Anyway, can you say at least one real argument against removing RS under checkpoints ?
It isn't possible to independently verify the chain. This significantly elevates the trust model for checkpoints from choosing among valid chains to trusting that the chain below the checkpoint was valid before being trimmed. Again, tradeoffs....
The chain is verified by the fact that the block hashes are immutable. An attacker can't change the transaction data by even 1 bit and get congruence with the block hash history.
The block hashes don't include the ring sigs. That's why the ring sigs can be removed without breaking the block hashes.
There is no assurance that there were ever valid ring sigs there, unless you were around to see it.
Among the trust assumptions here are that there weren't bugs in the code in the past that allowed invalid ring sigs to be accepted. But there are actually quite a few subtle assumptions that sneak in when you can't verify the entire chain.