The problem is not that the system is constantly under attack. It's that it can no longer be trusted to be secure. Nobody with money on chain will say 'oh well, probably nobody will steal my money today'.
It absolutely does, just not directly. Say that you have 100k fiat equivilent in monero and I demonstrate a successful monero double-spend attack. How much do you think your monero is worth?
The only reason this seems like an unusual definition to you is that you haven't thought about it very hard, yet. The critical thing is not how many XMR you have. It's what value those XMR carry. If I create from thin air 1000 XMR and it's known to the market I didn't just 'make' a certain amount of value, I made myself richer by making a lot of people less so. I stole value from them and gave it to myself, just like someone stealing a ham at the market. The only difference is that instead of burning one ham of value for one ham of benefit, I burn many ham of value for one ham of benefit.
Depends what the goal is. A state that wants to break the anonymity of the system doesn't care about $75m per day, specifically a state that can just print that...
This is how proof of work systems operate.
They are very expensive to attack but very cheap to recover from.
$75m per day is clearly unstainable.
Soon they will give up and the network will recover cheaply.
The attack is more of a nuisance than the end of Monero.