The relevant JIRA is SOLR-12196, but we felt this deserved a greater discussion.

Basically, our Admin UI is currently using AngularJS (1.x) as its
Javascript framework. That particular library has reached its
evolutionary end long time ago and is about to stop being supported
all together. The later versions of Angular carry the same name, but
are _very_ _very_ different. So, despite the heroic efforts that got
us here, we are facing this choice again. Also, as we were just trying
to migrate the UI and not rethink it, the underlying design/file
layout/plugin architecture is also quite problematic.

The question is what is the right thing to do next. There are
basically four options:
1) Migrate to the latest Angular in an incremental fashion (as per
JIRA's original proposal)
2) Jump to a different library (such as React or Vue) which got a much
stronger momentum and ecosystem these days, but sort of keep current
architecture (UI feel/approach)
3) Go blue-sky with new library and actually put some deep thought
into modern UI leveraging Solr's current features (Management API,
JSON, etc). Also, by picking React (for example) we get access to the
ecosystem of modern tools, extensions and even potentially mobile
4) Drop our own UI and adopt somebody else's (I don't have any good
suggestions here though)?

Normally, option 1 would be the sane one. The challenge is two fold though:
a) Even option 1 is a lot of work due the Angular team's radical
change of direction. Enough that it will lock us out from any other
option for at least another 3-4 years.
b) We are all server-side developers. So, even the simple Javascript
things are hard for us, never mind the CSS part. So, this makes the
cost of starting with any new approach dis-proportionally hard. Once
going, it is a bit easier, because the advanced concepts are similar
to other languages.

All of these, combined with all the open JIRAs on Admin issues - to me
- make option 3 less crazy than usual.

What do others think? Is there discontent with the current state of
Admin UI (apart from the implementation choice)? Are there secret web
designers here, willing to get us over a bump of migration? Is there a
company out there, willing to sponsor relevant skills to let us
leapfrog the incremental upgrade (similar to how we got the Reference