What happens? We get a hard fork?
Your node ends up with a stale block. It is just like as if 2 miners mined the same block at the same time but the entire network can only build on one of them and the other becomes stale.
Ok so if I download the Bitcoin software from 2015 and run it. Then I can basically steal all the coins from bc1q addresses. But the miners would refuse to include those transactions?
You can't "steal" anything when you can't provide a valid signature. It is like saying "I can modify the software and steal all P2PKH outputs"! It doesn't matter if
you think a transaction is valid when it really isn't and the rest of the network rejects it.
So let's say Taproot activates, but then majority of the non-mining nodes are too lazy to upgrade. And there still remain a sizeable number of miners who refuse to upgrade.
That is contradictory. A fork will not activate if it hasn't reached majority's support.