You don't know if the market is too small until you get out there and do some testing. Speculation isn't going to go far. If you have an idea and discover that

You don't know if the market is too small until you get out there and do some testing. Speculation isn't going to go far. If you have an idea and discover that someone else did to, great! It's been partially validated. Which particular edge of it are you going to take? What differentiates it from their execution? The subset of good ideas that have no current executors is way smaller than the subset of good ideas that can be successfully executed. Don't give up only because someone else is already executing; do it better than them.