Even before the AI era it was hard to find something to build in SaaS that wasn't built 10 times over.
Now it's much worse. With all the AI slop out there you need a really good differentiator, which is basically impossible because anyone can copy you in days. So the only real engine left is marketing.
You have to go all in on marketing, like Lovable, Gamma and others did. You need to create hype around your product, preferably before you even have the product. Start early.
You can also reach millions of customers through big companies, like what happened when Base44 got acquired and Wix fed it straight to their user base.
SaaS is a marketing game now, not a product game. If you actually want the product to matter, go build in a niche so unconventional nobody's bothered to slop an AI wrapper on it yet.