There is a high demand. Like exceptionally high. As a contractor I have endless work and never have a hard time negotiating my rate. There is such high demand because Salesforce is not as sexy as being a front-end dev that uses react or a back-end dev that uses go.
It's limiting and can be frustrating, yes.
Apex and aura components are not transferable directly, but what is is learning how to work with enterprise software. Plus, my bred and butter with Salesforce work is integrations, which has given me plenty of opportunities to learn other languages as needed to get the work done.
Sure, I've done that in the past. The problem with using a third-party framework is the longevity of the org. As a developer, I want to leave an org in better shape than when I started.
If I created an angular app here, a react app there, and elm lang app over there, you are dramatically increasing the overhead to maintain the org. IMO, it's better to play it safe by using Salesforce's tech stack so your client has an easier time maintaining their org in the future. Future Salesforce devs will thank you too :D
Plus, there is a very high chance that future interviews for Salesforce dev gigs, they are NOT going to ask if you know React/Angular. Nice to have skills for sure, but they want to know how quickly you can get started working in their org. And chances are their apps are built with apex, Visualforce, or aura components. I'd rather spend a year building with aura components to make my new project transition easy, than spend a year building with react and have to pick up aura components all over again.
Get a couple certifications. Once I got 3-4 and a few years of experience, recruiters approach me on regular basis. FYI I have no college degree. Companies don't care. Certs and experience are worth more than any degree in the Salesforce space.
It's limiting and can be frustrating, yes.
Apex and aura components are not transferable directly, but what is is learning how to work with enterprise software. Plus, my bred and butter with Salesforce work is integrations, which has given me plenty of opportunities to learn other languages as needed to get the work done.