I open LinkedIn and someone is already doing the exact thing I was about to do. Doing it well. With a bigger audience than mine. And the doubt lands instantly: I am too late, the good spots are gone, why bother.
The market is not a set of chairs
Attention is not a finite row of seats that fill up and then it is over. New people arrive every day who have never heard the thing said in your words, from your angle, by someone who sounds like you. The person who is already famous for it does not reach them. You might.
Your angle is the moat
I am a full-stack developer with eleven years in marketing, building the product, the audience, and the distribution at the same time. That specific combination is mine. Someone can copy my topic in an afternoon. They cannot copy the exact path that got me here or the way I see the problem because of it.
That is the part that does not get taken. The topic was never the scarce thing. Your version of it is.
The takeaway: late to the topic, first to your version of it. Those are not the same race, and only one of them matters.