From the technical side, maybe. From the user side they could not be more different. BeOS was the ultimate power user OS. You had a lot of power, but also responsibility. It included many features like the novel filesystem that more pragmatic OSes didn't include.

Fuchsia, as far as I can see, is the ultimate "modern" OS. While very safe, it will be very locked down, in the tradition of iOS, Android, Secure Boot, HTML5+js etc. but "done right". There will probably be a lot of things I won't be able to do with a Fuchsia phone.