If it is to learn something, the answer is obvious: build it anyway. You will still learn pretty much all of the things you otherwise would have.
If it is because you feel that the world needs this thing to exist, and it'll become a better place because it does: see if you can contribute to the other people's work. If not, see if what they're doing meets that need and be happy that other people shared your vision of a problem to be solved.
If it's because you wanted to make a company and earn a living: keep going. There are thousands of niches in businesses. You can find yours.
If it is to learn something, the answer is obvious: build it anyway. You will still learn pretty much all of the things you otherwise would have.
If it is because you feel that the world needs this thing to exist, and it'll become a better place because it does: see if you can contribute to the other people's work. If not, see if what they're doing meets that need and be happy that other people shared your vision of a problem to be solved.
If it's because you wanted to make a company and earn a living: keep going. There are thousands of niches in businesses. You can find yours.