As more changes are implemented and forks created, the harder will be for your Mempool to estimate fee's.
Also, you running Knots won't stop the "Spam" from happening or taking space in your node SSD.
Discussion
the mempool fees maybe, but what do you want to do if people just dont upgrade? you can run a Core version almost as old as you like, do you want to do like ethereum, any change is an HF so you accept or fork off? If running Knots doesn't work so whats the problem? let people run what they want bc that was and is already the case.