Additional capacity (in terms of connectivity to the node) gets added all the time. But whether the individual ISPs keep up with pace is another question!
I think it's the second part of your answer that is the more relevant; ie the ISP has to have sufficient MSIL capacity that is usable at a non-penal rate for exceeding the contracted amount.
However the Entanet problem was very much down to them going for WBC not WBMC, plus a BT policy decision at the last moment to do excess usage billing by the node, not overall. Allied to the fact that individual MSANs can be routed (on a fixed basis) to non-obvious nodes and not being able to get details of that routing from BT.
As a result they had an almost impossible task finding the correct capacity at each node.
ISPs using WBMC don't have that problem. They just need to contract for enough.