If that happens, we will switch to a stronger ECC. For example from 256 bit to 512 bit.
It's more likely we will switch to different cryptography though, which likely to be quantum resistant while remain compact in size.
Exactly. Anything that could break ECC, for example a solution for ECDLP that takes reasonable time, won't be solved by increasing the size. It would only postpone the issues by a very short amount of time. In other words if 256-bit curve using ECC were to be considered weak it will only be a matter of time before 512-bit ones are considered weak.
Take your SHA1 example. It is not weak because of its size (160-bit) although that plays a small role. But it is weak because of its weak algorithm which means even if we extended the version one Secure Hash Algorithm to be 256-bit it still would be weak. Instead we use an entirely different algorithm (ie. version 2).