If your argument was true then the NFL would never consider going with Apple TV+ because they have a fraction of the subscribers Amazon Prime does.
In reality though your claim is ridiculous - there would just as many NFLST subscribers if it was on Apple TV+ as there would be if it was on Amazon Prime despite having so many fewer subscribers. Just like hardcore NFL fans today subscribe to Directv to be able to get NFLST, tomorrow they would subscribe to Amazon Prime or Apple TV+ - at least for the months September - December. It would save them a bit of money if it was on a service they already had, but if they're willing to pay $300 for NFLST they aren't going to balk at having to spend an extra $5 or $10 per month every fall to get the service that allows them to subscribe to NFLST.
While Directv costs a lot more and doesn't allow subscribing for just a few months, the kind of hardcore sports fan that wants NFLST considers cable/satellite (or equivalent service like Sling TV) a MUST HAVE. There are tons of live sports that are simply not available via streaming. There's little difference between the services, Directv may be a bit more expensive than some but not enough to dissuade someone willing to spend $300 for out of market NFL games. So I just don't buy that there's a huge pool of potential NFLST subscribers out there for the NFL to find. Most people who want it, have it.
The only gotcha is that neither option is available everywhere. Some people can't get Directv where they live, and some people can't get internet good enough to do live streaming where they live - or are older and less technically adept and would simply feel it is beyond them. Whichever way they go, if the NFL does an exclusive they are going to keep some potential customers away.