This "anyone can spend" concept is interesting.

I read on Jimmy Song's blog that before Taproot is activated, if I transfer coins to a Taproot address, then anyone with or without the private key can just take those coins.

So.... let's say there's a bunch of people running non-mining full nodes that were installed in 2015 and never updated. They use this "anyone can spend" rule and publish transactions stealing coins from bc1q addresses.

A mining node running 2015 software mines a block containing those transactions. This block gets accepted by non-mining nodes running 2015 software, and rejected by non-mining nodes running 2018 software.

What happens? We get a hard fork?

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?

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. Then a non-mining node publishes a transaction stealing Taproot coins using the "anyone can spend" rule. And this transaction gets mined by a mining node, and the block is accepted by non-upgraded non-mining nodes. We get a hard fork??