It seems like if we're going to guess what you name Milestones, we should just parse you're previous title and add 1? Or I'm ok just removing the feature outright. I do like the idea that some simple heuristics could be applied.
v1 -> v2
Sprint 1 -> Sprint 2
Milestone 1 -> Milestone 2
Week 1 -> Week 2 ?
I don't think we can guess right for stuff like "Version 6.5" or "Sprint 4/31" without doing a lot of work, since "add 1 to any numbers" won't work in those cases.
Removing it outright and adding a "Previous Milestone: Sprint 16" field (as a hint about which milestone you're creating) might be good for now.
I'd vaguely imagined letting you set some kind of template on the parent task (Milestone Names: 'Sprint %N'), but that's wildly complicated and not meaningfully better for "Version" cases and still tricky for date-based cases.