"Replacement cost" has a specific meaning in a finance context. It means market price of an asset
in the same state & condition as the one being replace. Not a better/newer one.
Hate to break it to you but that house was not worth 2.1m when you picked it up (might be now though). As you said the bank sent an assessor. He works
for the bank. The bank cares about only two things: Extracting maximum interest from you and minimizing the risk of them not recovering their investment if you fail to pay.
These two goals are in conflict with each other:
Max interest->Lend as much as possible.
Minimize risk of disaster -> Lend as little as possible against as much collateral as possible.
Within the bank these are two distinct function: The one is handled by risk management, the other by sales. Both get performance bonuses.
The easy (& bad) solution to this is to inflate valuation numbers:
Risk management is happy: They've got 1.6m debt secured by a property valued at 2.1m. Yay! Risk management dept gets a bonus.
Sales is happy: They managed to sell a 100% mortgage and get lots of interest. Sales dept gets a bonus.
Smiles all round.

Doesn't matter that the 2.1m doesn't reflect reality. It allows the banks to take higher risks while pretending that they are playing it safe.
Interestingly due to the way foreclosure works, the risk management actually works out fine despite the 1.6m vs 2.1m difference (for the bank) as long as the buyer manages to hang in there for say 2-3 years.
The truth is unless either the buyer or seller is an idiot, purchase price = market price = value. In the absence of market distorting forces (idiocy, pressure to sell etc) that will always be true.
As for the 10 year issue: I was talking about what it should be insured for initially. Nedbank wants to insure a 1.2m house for 1.5m. The house is then over-insured. That is very bad because you pay higher premiums but get zero benefit (since the pay out replacement cost, not insured amount). Ideally you want the insured amount to always match the current value of the asset, to ensure that you are neither over insured nor under-insured at any point. You do this by constantly updating the insured amount.