I have personally witnessed companies take guys who have been slapping together "apps" in jQuery / .NET for the past 10 years and tell them "it's time to make the stack modern" and "we're going with Angular because we're a Microsoft house".
Things go very bad very quickly.
If you don't take the time to understand the Architecture behind scalable modern web applications you're going to have a bad time.
Anyone can slap together a quick and dirty app with Angular or React - not everyone can scale one correctly with thought given to the architecture behind these applications.