Using rails-api as a backend with your choice of frontend is still a viable option. Perhaps it’s not one’s first choice these days but it’s far from obselete.
I have three languages I use. My web sites are done in Rails because it's fast and logical. One site has a Vue front end, only on a couple of pages where I need the JS fancy stuff. Most of my pure APIs are done in Flask because it's lighter weight. I use React Native for my native apps. I tried using Express.js for the APIs backing these apps but I could never get it all working right. I ended up rewriting it in Rails and now it's rock solid and fast as hell.
I'd also like to mention most web development out there is still right in line with even old school PHP and does not need React or Angular or anything fancier than Rails. YAGNI.
Devise does a great job of it though - https://github.com/plataformatec/devise
https://api.rubyonrails.org/classes/ActiveModel/SecurePasswo...
Because on production servers we have 10 Ruby front ends for a single Postgres database that barely sweats. At the same time, we have a lively Hibernate setup in crufty java using 4 front end servers and overloads our database regularly.
Ruby is slow as hell, to the point that it costs a lot more money to run servers vs ancient J2EE junk. V3 speed increases will be interesting if it's 10x+ . If not, Ruby is dead. Even Python and it's crazyness is far faster and somewhat easier to use.
Ruby is doomed
For a lot of people (and businesses), Ruby more than suffices. If it doesn't, other languages always exist. So no, Ruby is far from doomed–one just needs to evaluate the pros and cons of Ruby vs something like Go, etc. Ruby with Rails isn't hype or sexy technology, but it is quite mature and robust.
Sometimes Ruby's faster, sometimes Python is.
However all those benchmarks are probably useless. They are not testing how the language is used or should be. The trick with all the scripting languages is to make the CPU spend most of its time in the C code inside the standard library or the implementation of the native constructs of the language with as little overhead as possible.
Example: you don't want to do numerical computation using plain Python, you want to use Numpy.
You also want to learn very well all the ways Ruby's standard library can group operations in a single method call saving time compared to naive multi method chains.
When it reaches 1.0 and becomes stable I’d like to work with it again. For small simple APIs raze was great.
There's too much magic and Ruby is seriously sllllooowww. Slower than PHP, Perl, and Python. The main draw of Ruby was rails, which is functionally obsolete for modern front end frameworks like React and Angular. It's the OSS version of Asp.Net WebForms