I don't think a visual regex debugger is a tiny market- it's every programmer who doesn't totally understand regex, or who will have regex that he needs help decoding. This is probably close to 99% of programmers.
If his market is "programmers who will pay for a regex debugger", it's probably much smaller than 99% of all programmers (and as a sibling comment pointed out, "programmers" are a much smaller market than many other).
Yes it's a huge share of programmers, I'd say 80-90% for your first example 95%+ for your second. But compare that market to the market for Dropbox, or AirBnB, or Amazon.
If you're market is "programmers" it's already a pretty minute fraction of the general population.
