> Rails was designed at a time before angular brought the SPA into fashion
I dunno that Angular is the relevant point here. Rails (late 2005? I think?) post-dates the public existence of Gmail (public April 2004), a wildly popular SPA. It also post-dates the existence of Google Maps (early 2005). The famous blog post by Jesse James Garrett that coined the acronym "AJAX" is also from early 2005.
The thing that Rails does well was already largely-obsolete before Rails was open-sourced. (Though that is one state-of-the-art buggy whip! Very impressive!)