you forgot the risk, that a mined block is invalid. because nodes will check signatures and might not accept the mined block as a proper block.
even if a miner finds a block, but accidentally a non verified sig is inside, this block will not be the main chain.
You make a good point and hopefully the development team coded around this issue but all open source code can be
changed and I am guilty of using a very short, quick and fast bit of encryption code that would be hard to crack was
it not for the code being open-source.
Segwit seem like a good fix to buy some time for me if only they didn't break existing wallet code or if they did then
converting to a 03 address should had been free from miner fees.