This is a bad idea. If you look at the list of advantages to doing this only 1 is user-centric (things would be reactive). The rest are all related to how it makes development easier. When you choose a developer-centric workflow your users will suffer.

You only have to look at the numerous server to SPA conversions to see how consistently bad of a choice this tends to be; Reddit is a big and obvious example.

They could take a half-measure and move away from the brittle jQuery based front-end they are currently using by adapting those to use something like Preact, while leaving the rest of the page alone. This would give you more dynamic and more maintainable page widgets without the sacrifices that inevitably occur when you move the entire site to being front-end rendered.