I think you insulted me, but I'm not sure. I'll respond regardless.
You didn't ask about how to judge the value of different ideas, you asked how big a niche market had to be for SaaS to work. The answer is that there is no pre-defined minimum niche size, so long as you can be profitable. The key part of being profitable, assuming you're bringing in any income at all, is managing expenses.
It is completely irrelevant and next to impossible to gauge whether one idea is more or less profitable than another. Some comparisons may be obvious, but spending too long figuring out what idea stands the chance of making you the most money is wasting your time.
Find the one you think might be the best, and built the thing. Rinse and repeat until profit > 0.
