This simply isn't true. Every year since the present day ML wave started has seen more and more domains tackled. Even something like that silly lego sorting machine I built could be the basis of a whole company pursuing sorting technology if you set your mind to it. And that's just resnet50 in disguise, likely you could do better today without any effort.
Your statement reminds of 'all the good domains are taken', which I've been hearing since 1996 or so. Of course you'll need to do some work to identify a niche that doesn't have a major player in it yet. But the 'boring' niches are where a lot of money is to be made, the sexy stuff (cancer, fruit sorting) is well covered. But more obscure things are still wide open, I get decks with some regularity about new players in very interesting spaces using thinly wrapped ML to do very profitable things.
