Its not up to ISP's at all, in the end they are all resellers(unless they saved up enough to lay their own cable and even then they can't resell easily on e.g. ADSL platform). The people with all the marbles are Neotel, Telkom, Vodacom, Mtn, Iburst, IS uhm.. who else is there? Basically if you provide the underlying network, you control the pricing. (So ADSL pricing might not drop, but wifi providers will be able to tackle it.)
What could happen though is people sourcing bandwidth wholesale from there directly such that wifi operators will suddenly start flourishing and putting pressure on the above guys to drop prices on their consumer side of things. But then all they have to do is make the cable connection cost to seacom 'gold' and nothing will happen