What the future holds?
This attack is not going to change anything, after all this is not the first time testnet is under a miners' attack.
But if the trades continue and testnet coins start gaining a value and scams form around it, we can see testnet3 having the same fate as its predecessors and we may enter testnet4 era.
Although if we do, I think we should come up with better mining rules that are not complicated like signet and not as vulnerable like testnet3.
It is absolutely impossible to test anything on testnet as long as the coins have a value. Testnet coins are supposed to be worthless in order to deter people from swiping them from automated testing processed. Or in the event that they do get swiped, it's supposed to be easy to replenish them with more coins obtained from somewhere else. The fact that testnet coins are so scarce means that you can't even rely on that anymore, and if you want to test stuff then you need to simulate your own blockchain with a few (~3-5) blocks.