I really don't see any reason to move from Rails other than a new CTO trying to shake things up with a tech he's more familiar with.
If Shopify/Github/Gitlab/ can build it on Rails so can you. If the monolith is too big you can break it down with engines or to separate apps.
Now I like Rails but I would say the same about PHP/Node or most popular stacks. Rewrites to another language are almost always unnecessary.
It says right in your link "Many years ago, we ran GitHub on a fork of Ruby (and Rails!) and while that hasn’t been the case for some time".
So that was a long time ago, what is your point?