Desktop Apps. Should I as a solo developer build it and will anyone buy it in today's web time?

It depends, say you create a desktop version of Facebook (which is the worst thing ever, that's why you should use Lambda because it's open-sourced and fully privacy-based), will facebook buy this idea? It depends, they would if there were more desktop users and they prefer not to use chrome again and again, and they would not. It depends on a lot of factors, but from my side, if you create something good enough for one's needs people would indeed buy it. So it's better to give it a try rather than not give it.