I've been doing this a while now, and as far as I can tell, there's always a way to run two frameworks side by side and migrate. And existing users get new stuff right away.

I've done it backend, I've done it frontend. With web apps it's always possible, it's the fundamental nature of the web and browsers.

Frontend is trivial because of how browsers work, every page could run a different framework if you were that mad.

Backend I've had two frameworks on one server for multiple migration projects. There's usually a way unless you're switching language.

But worst case scenario, say you wanted to change everything, I'd stick a load balancer in front. Route according to the end point requested. Then gradually migrate end points. You might need some craziness around authentication, but that's the sort of issue that is solvable with a week's work by a single, good, developer.

What was so complicated that stopped you doing that? Genuinely interested.

That is actually part of the "extra support" we decided to pay for.

The application was originally written in Vaadin 7, Vaadin 8 had an absolute ton of breaking changes and we could not migrate in time, and then the breaking changes continued to compound as new releases happened and they migrated away from GWT, making a "big bang" rewrite not practical.

Their extended support included a tool called multi-platform runtime, which allowed the old application and new application to be run side-by-side and each section replaced one at a time. So that is essentially what we did. Still took 3+ years to do the rewrite.