This ticket should depend upon that work.
I disagree, but not enough for a formal -1 vote. These admin UI improvements are pretty important in my opinion. I think we should expose the Collections API for SolrCloud collections and keep using the CoreAdmin API for cores until the non-cloud Collections API is done. If the new API is done right, it will be substantially similar to the existing CoreAdmin API, so the admin UI migration should be very easy.
The CoreAdmin API is not going to disappear, at least not in any 5.x version. It just won't be publicly documented any more, and users will be encouraged to switch to the Collections API. Some deprecations in SolrJ are likely.