There are "special" things themes are doing that block-based widgets are not going to provide continuity for. I hope the Customizer, widgets, etc. are not deprecated. Existing themes need to continue working exactly as they do now, indefinitely.

Otherwise, really, WordPress ought to have been rewritten from scratch rather than Gutenberg being worked into the existing project. I'm worried that backwards compatibility is not as important as it has been and that that's going to be very inconvenient for site owners and theme maintainers.

I am very excited about FSE, but only for brand-new themes - not existing themes.