Proof-of-stake, and decentralised systems in general, aren’t about guaranteeing the behaviour of individual entities.
I agree that the barriers to entry shouldn’t be too high for NPs (or it would hurt scalability). But scalability shouldn’t be prioritised over robustness. I think you could have both by having two tiers of NPs. As longs as more than 1/3 of the nodes in a subnet are backed by a large proof-of-stake, then you have a significantly more robust subnet than one where the nodes are not backed by a large proof-of-stake.