The practical reason to ask that question is important. But it doesn't factor in the very real human desire to create. Would the band you see gigging around your local pubs stop practising because other bands in their genre already have record deals? Would a dancer stop dancing or a writer stop writing just because there's competition?
The truth is, you won't know if you can make a living from your art until you put in the practice.
Here's another practical point of view: if you invest the time to develop your skills and style, you'll be in a stronger position when the industry is booming again.
There's always an ebb and flow to demand in the creative industries. While it might take you a bit longer to get your foot in the door now than it did a few years ago, there is always room for dedicated, skilled illustrators. If you want to be an illustrator, now is the perfect time to prepare yourself for your future career.