How to develop UI framework agnostic?

Question

JavaScript

After the release of vue 3, it became obvious that the developers there don't give a damn about those who use their product, which I already knew for a long time.
The question arises, how to develop UI framework agnostic code?
I wanted to replace vue with react or upgrade versions that are completely incompatible, then the time spent should not exceed 10% of the time spent on development.
Or use instead of Vuex, Redux and that's it, a dead end.
Here the company spent 100,000 hours writing components and all their UI on VUE 2, and now we need to spend another 50,000 man-hours on porting to VUE 3. When even 10,000 will be too much.

UPD. I repeat, the essence of the question is how to develop a modular UI without explicit dependencies on the framework but with the ability to use components, features, frameworks, packages. This is how you install the library, it should not affect the main code. The question is not about upgrading or switching Vue / Angular and more.
For example, in the back-end, you can easily use both symfony and other components of other frameworks in one application. How to do this in the UI, I do not understand yet.

PS, This is just an example, the numbers are completely invented and everything is different for each project, but still, it is costly.

Answer 1

Nobody canceled Vue 2x, it is not dead and will not become obsolete soon. If you don't like Vue 3x (by the way, it's not a PR yet, just in case), then use Vue 2x, you don't have to rewrite all projects. Especially with the cost of 100k hours.
Reply to UPD

how to develop a modular UI without explicit framework dependencies but with the ability to use components, features, frameworks, packages.

There are a huge number of UI libraries. Including with all sorts of shadow DOM bells and whistles. This allows you to get away from using the framework to the side, but you pay for this by the need to configure the ecosystem.
Basically, it depends on the specific case. If something complicated is not needed from the UI, then the approach with a cascade of UI libraries is in principle suitable.
As for modularity - set up webpack and saw modularity with all the conveniences of babel and other packages.
The simplest minimally working webpack configuration takes up as much as one line. This approach is somewhat reminiscent of React, by the way.
But this is such a headache that you better relax and calmly saw on Vue 2x and not know grief.
You don't fully understand what you want. You want "UI with no explicit framework dependencies but... framework features". You want a framework without frameworks. Like making money without making money.

Answer 2

So what's the problem then? Ditch all frameworks and write everything yourself using pure HTML, CSS and JS.