Well, learning Slim will improve your skills (the concepts it uses will be used by any modern framework), so it's a good investment anyhow.
As for being economically viable, I've switched to Slim for most my programming jobs and am doing fine with it. Having said this, I focus on custom/complex implementations where the choice of platform is a lesser concern than if you have to implement a stock company site. My customers are happy I can provide a solution and don't really care about the platform used.
Most PHP jobs are platform specific, so from that perspective Slim may not be the best choice since you can't really propose to use Slim if the customer wants "A new corporate website based on Laravel/Wordpress". However, if you can break out of that mold, Slim is IMHO a great choice.
Bottom line: it all depends on your ability to find the right type of jobs and market yourself properly to your customers about being able to deliver. I'd have to agree with @akrabat that you don't want to be a Slim-only guy ... learn a number of frameworks and you''ll be able to find work easily as long as you can deliver a quality solution.