Actually right now Flutter Desktop support is officially in "Alpha" not beta, so sure its immature compared to Electrons ecosystem foro example. But on the other hand, is far more mature than day Jetbrains Compmose Dekstop effort.

But I have no idea what you mean by "prehistoric support" ? Was that a typo?

Also rewrites, especially just to change platform are often a very bad idea. But if you are writing a new app or radically reworking an existing app, using Flutter instead of Electron today would have some merit.