My point is rather if you are doing that (a hybrid app) wouldn’t doing a full spa actually be simpler? And allow you to use a less complex framework / a framework which doesn’t have to support that legacy.

You're going to need to define a "full SPA". Every SPA needs a backend, and Rails is as good as any. In fact, Rails + Vue is, by any definition, a "full SPA" as Vue is a SPA front-end framework and SPA only refers to the front-end design. You can launch Rails in API-only mode so there is no "legacy" anything, just a backend framework that has all the benefits of Rails while still giving you the SPA front-end you're looking for.

I wouldn't even call it a hybrid, nor would I say it's any more complex than anything else. Every SPA needs a backend, and Rails is as good as any.

By full spa i meant something where all ui is rendered on the client side and the server only serves static files and a set of web services for whatever business logic that has to run on the server.