Well, with ~60 BNGs you need 6 bits to subnet for them, and going smaller than /27 subnets (29 usable out of 32 consumed, wasting 10% of the IPs on subnetting) per BNG really doesn't make sense and creates operational burdens on their side.
And with L2 hand off, there would be no wasting, and no problems, and no (almost) obscene requirements or wastage

I do get where you are coming from though (I also only found out earlier today that there's that many BNGs actually).
And if you have fewer than 2000 customers on DSL+Openserve fibre, you're not going to be buying big enough volumes (>10Gbps) of IPC to be competitive, and you should rather re-sell (e.g. IS or similar).
And be dictated to in terms of what I can, and can't do, or what my product should, or should not entail, have no control over it? Have even less insight into the network (as if DSL troubleshooting isn't troublesome enough with all the congested exchanges), sit and argue with the wholesale ISP *and* OpenServe about unacceptable congestion and/or contention? No thanks, I think not. OpenServe (or Telkom) made IPC over complicated enough as it is. No need to have even more fingers in the pie.
Then there's also of course the fact that my clients sit on IPs registered in other's names (and AfriNIC being so daft, that they actually discourage ISPs to register sub allocations in the actual client's name?). Would others, be willing to pay me for the free advertising they get from my customers, ranking their IP blocks and networks positively on services such as speedtest.net (for example)? Or would that just be yet another one-sided benefit which only benefits the "big boys". Another way to crush, hide, and discriminate against the smaller guys?
You'll be surprised at 1) how many customers ISPs lost, and 2) how many DSL lines OpenServe lost over the years, due to OpenServe's congested exchanges. But, I guess you know that too... In fact, isn't OpenServes fixed lines pretty much at a all time low? It's all sunshine and roses at the exchanges yes. We can trust OpenServe to carry our IPC traffic without any congestion.
For a long time actually, I had the stance where I wanted to flat out refuse to purchase IPC (not for DSL, not for FTTx) due to lack of control / insight into IPC from OpenServe - but alas, unfortunately it looks like (for now at least), there just isn't any viable alternative. If you want access to the market (i.e. client), you just don't have any choice really.
Based on pricing I received for IPC (orders going in soon), I could be rather profitable with as little as 500 customers (and fairly competitive too). Not everyone has millions per month in overhead costs that needs to be covered... Not everyone has the intention to become the new WA, or AH (heaven forbid). I do appreciate your views however. There's nothing wrong with being small and profitable.
Yeah, maybe lack of progress on IPv6 is a problem. Vumatel didn't have any idea of when they would be ready when I asked them. Openserve was making progress on some blockers for IPv6 on IPC (there are some technical limitations on some network platforms compared to IPv4 that meant they had to re-architect some products and migrate customers etc.). If you didn't heed AfriNIC's warnings and get your address planning in order last year, yes, you may struggle to get enough IPs ... but then for now you should be a reseller.
And yet all our L2 services for which we run our own BNGs, are fully IPv6 enabled. We successfully allocate /48s (by choice) to our customer's CPEs and 30% to 40% of all our customer's traffic is in fact, IPv6... Hey, we we can do it (who apparently "balk" at R150k), surely OpenServe can do it which has billions?

I've heard rumors that it's coming yes - I do look forward to it. Hopefully you don't require a /29 or something crazy like that from the ISP to operate it though.
OpenServe cut it's own throat be providing a L3 service on IPC, and not a L2 service... That, unfortunately is the true problem. With a L2 service, I can happily be profitable with my 500 customers, a /23, and 1 or 2 BNGs. If you can't provide L2 for whatever reason, then there's also L2TP PPP tunneling which is quite the common standard elsewhere... OpenServe is dead set and fixated on controlling the BNG, and it's wrong. The BNG should lie in the ISPs realm, not the infrastructure provider's realm - just my 2c. The fact that many, many open-access FTTx providers operates on L2 services, and the ISPs therefore, very successully, runs their own BNGs just proves my point to. No silly /21 requirement to resell Vumatel, Frogfoot, SADV, <insert your faviourite operator here except for OpenServe>
Who knows, I may have no desire to have 2000+ customers. Large customer bases means large amounts of support, means large amounts of overhead. Sometimes, being small, isn't always such a bad thing

It's just a pity you don't always get treated equally (read, oh you're not big enough - I won't peer with you in ZA, but I'm more than happy to exchange traffic with you at International IXes yes). It's scary, how smaller ISPs are being discriminated against in ZA, compared to the rest of the world.
And YES. OpenServe routes ALL our traffic (currently) through AMS-IX where we do peer, because they refuse to peer with us locally in ZA. Not us, OpenServe... This, is not a figment of my imagination, it is a set practically, happening right now. IS (another company with a bs preferential and unfair peering policy) did the same - we did since manage to get them to at least route to us via national routes, instead of international routes. And all of this... Because we're to small... *sheds a little tear* It's blatant discrimination that you are prepared to peer overseas, but not locally! Protecting your turf, as they say, eh?
Even without last-mile infrastructure costs, the ISP business is a volumes game and requires a lot of investment. If you balk at R150k for licences, you have big problems ...
I suggest you read what I typed, again. OpenServe is the one FORCING the ISP to go after volume, rather than quality due to the requirements put in place to operate a ISP in this country. It's not by choice - trust me on that one.
The ONLY reason, why there's so few ISPs with their own IPC, is due to *OpenServe's* unjustifiable requirements for it. Price is long since, not a problem anymore (I have the pricing - it's not that expensive). It's the things that OpenServe REQUIRE from the ISP, which is expensive. OpenServe, is (and always has been) creating the barrier to entry.
I can recall complaints about the absurd (/21) requirement for example for as long as IPC existed, I can recall complaints about the lack of being able to use your own BNGs for as long as IPC existed... These aren't new problems. They've exited for years. Instead of making things easier, things are becoming harder due to (among other things) the lack of available IPv4 space.
PS: This isn't me attacking you (or OpenServe) or anything. This is me, just telling you what the reality, really is like... The OP wanted barriers to and challenges to becoming an ISP... Well, this is it.