Most sufficiently complex codebases that aren’t “framework” based will have a poorly implemented, feature-lacking, bug-ridden version of a framework eventually.
If Django doesn’t fit the use case for your thing, don’t use it? Just don’t use flask and then start reimplementing Django.
Also, Rails is better Django. It’s not even a close comparison.
Every flask REST implementation I've seen has been a bespoke library combinatorics mess compared to something that could have been written in Django REST framework.
It’s impossible to predict the future. I would rather start with rails and rewrite portions later on, than start with Sinatra and shoddily reimplement rails features.
I also just think Django is terrible compared to rails, and that may be why the author has such a dim view of frameworks in general.
It’s worth noting huge companies started with frameworks initially. Just looking at rails: Twitter, Shopify, Airbnb, Basecamp, ..
If Django doesn’t fit the use case for your thing, don’t use it? Just don’t use flask and then start reimplementing Django.
Also, Rails is better Django. It’s not even a close comparison.