Suppose the majority decides that they won't halve coinbase (the mining reward) after every 210,000 blocks anymore (thus removing the 21m cap). Maybe that's irrational, maybe that's rational. Who is to say?

Right. Though the problem is not so much behaving irrationally as dishonestly. At any rate it's hard to argue that the bitcoin network is more "trustless" than a bank, since we have coercive mechanisms to ensure that a bank will behave honestly, while we don't have the same mechanisms to ensure bitcoin miners will do the same. Bitcoin users have to trust that they will be honest.

No, you still don't understand. It's not dishonesty, it's irrationality.

First you assume that the majority computation power is already colluding. This is not the case.

Let's assume it's the case. As soon as they stop adding all transactions to a block that they would've been expected to, the entire world will notice. In response, the currency will become valueless. That is, it's a public act to abuse your position as a majority power, and the very act of your abuse destroys the thing that you're trying to abuse to your profit.

No, it's you who don't understand. There are many ways someone can profit from bitcoin losing all its value. And there many motivations other than financial profit. Therefore it's more accurate to speak of dishonesty rather that irrationality.

This is key. Profits from Bitcoin or from Bitcoin's losses are not restricted to the Bitcoin ecosystem. In other words, the Bitcoin ecosystem is not closed. For example, Ethereum or Algorand or Avalanche may benefit if bitcoin loses its value, even if those communities may not participate in Bitcoin.