Picking the right technology is hard. Finding the right balance between productivity, speed, and developer knowledge is never perfect the first try. While I've heard it said that rewrites are bad engineering it is often the case that rewrites are a normal part of optimization. It's risky because it's totally possible that a rewrite is worse (See reddits new UI). But fixing existing code usually only makes it better.
In general to get a job programming the best way is to look at the job market in the area, find what what technology people are using and master it.
If you live around oil and natural gas people tend to use C#. Banking people tend to use Java. Cloud people tend to use python and golang and docker.
networking/ai python.
Mobile development it's good to know either android or ios. if android both kotlen and android because jobs maintaining older apps exist.
React is basically the defacto ui. I'm not a huge fan but it has job security.
Twitter uses several custom stacks. They wrote bootstrap which is the most popular UI library in the world.
Twitter uses a bunch of Scala on the backend. There are really few engineers who have worked professionally in Scala and that is a decent argument to rewrite it. Twitter has probably written most of the Scala libraries out in the wild.
If they just used Java they could reuse the libraries everyone else is using. Not that I love java but all the boiler plate code is already written and tested. It's also infinitely easier to hire for Java than Scala. Almost no university teaches Scala but all of them teach Java.