And that cost will be paid only by upgraded nodes, so they can do that, if they want to. The rest of the users will still enjoy 4 MB limit. And then, if it will turn out, that all quantum FUD was unjustified, and there is no danger, then these new nodes will have their own bloated version, which they voluntarily picked, so they can leave legacy traffic unaffected.

Only Segwit nodes are affected by Ordinals. Old, legacy nodes, see smaller blocks in practice, when witness space is fully filled. And the same will happen, if Ordinals will move their data into quantum signatures: we will see smaller than 4 MB blocks in witness space, which is good.

It is possible to use more than one layer of verification. Currently, there are at least two: one is legacy data verification, and another is witness data verification. If we would have a third layer of quantum commitments, then Segwit nodes will be unaffected, and process only ECDSA signatures, without checking, what is behind R-value of each and every signature.

So, to sum up: if quantum enthusiasts want to have enormously huge blocks, then it is their choice. If they raise it by too much, then only their nodes will suffer, so we shouldn't stop them from shooting themselves in their feet. And if one quantum proposal with big signatures will lose competition with another quantum proposal with smaller signatures, then again, it is their choice, if they want to consume more space, or if they want to spend more time on verifying smaller signatures. They should compete with each other, so that Segwit users will be unaffected, and they will pick the final winner, if we will see, who did it in the best way.