I think some of Ramanujans results might have come significantly (more than a decade!) later without him, but I don't think that this would have held up mathematics as a field significantly.

For Cantor, I think the maximum delay in results would've been even less, but maybe the impact (=> set theory) bigger during that time?

I will concede that there is a lot of value that any specific scientist provides just from correspondence/communication alone which is very hard to quantify.

I also think if you took away whole institutions from a hypothetical timeline, like, e.g. the University of Göttingen, then the impact could be quite clear (more than just a decade of lost progress in some field). So maybe you could argue that some founders of prodigious institutions helped human science more than any single scientist? But this is highly hypothetical (if the people AT those institutions would have existed regardless). Also a distasteful thought (to me).