Many (most?) dois when resolved on doi.org are merely pointers to another website which means that they already being oneboxed if the target website supports it:
https://doi.org/10.1017/9781316941591 for the box above.
Those that are not forwarded are not oneboxed because doi.org doesn’t seem to support OpenGraph:
For these urls it would indeed be great to have onebox support. So the question is whether the team would consider considering doi.org as worthy of it’s own oneboxing engine…
I wouldn’t say that the oneboxing feature would/should replace the doi-resolver plugin because you’d still want that in situations where a doi is mentioned inline, e.g. in a reference.