Well, kind of. Such an attack would grind the network to a halt for a time. But in the end yes, the miners would own all the capital they spent on it and their currency would be much weaker, they'd hurt themselves much worse long term for a short term attack.

It's all incentives, game theory. The main reason they really don't have a blank check is because of the way their fiat money works, and not due to how XMR or BTC work. They don't have a blank check to do anything, they have to manage their resources.

But yeah, the fee market is designed to deter spam of any kind at any scale. Every now and then you get someone that wants to burn a bunch of money but for the most part these blockchains are spam free.