Like others comments , Kudos to the authors.
I'm an SPA Lover ( Angular , Vue ) , but still most of the points raised here a coherent from a back end perspective.
That said , a lot of the arguments are basically summed up as :
"I don't like Front End Dev. because it's not as mature as Back End".
I will not lie on this point , JS is a fast evolving ecosystem that sometimes has issues to stabilize. Hence , there is a lot of marketing and self promotion mixed with frameworks sometimes ( Growth Hacking ) pushing for unnecessary tech that dies a few days after they have leaved the Github Trending page.
For beginners , when a framework reach a certain threshold ( Github Stars most of the time) they feel like they should hurry and use the framework to stay relevant while most of the time they don't need to and the tech is just fluff.
My point here is very simple, you are ROR or Django or ASP.NET dev and you love what you do ? As long as you find jobs keep doing this , don't bother with Full Stack Fluff.
Now that said , I can only encourage the author to do the Angular or Vue tutorial to discover something different. Rails is great ( it's empowering a lot of website Github , Airbnb etc... ) but SPA are different and when used properly it's hard to go back , the experience is delightful.
Most importantly we are getting closer to the serverless era where backend will be completly different from what it is compared as now, and SPA and SSR will likely be the standard in the future.