While I do not think this is the last generation - nor do I think they will ever intend there to be a "last generation". If Pokemon ends, it'll be because it stops selling. They're not going to choose to end it.
However, one thing that I do think will stop happening in a few generations is that they'll stop releasing new Pokemon with new generations. They'll make new regions, new regional dexes, to bring together Pokemon that have never been available together, new moves, new abilities, etc... But they simply won't introduce any more new Pokemon.
This'll probably happen gradually. Like, maybe Gen VII only introduces ~50 Pokemon, Gen VIII introduces ~30, and IX introduces none?
Let me emphasize - I do not think this because I believe they are running out of ideas. It is very obvious that they're not, and, even if they were, they could simply hire new people to make new ideas. I think this because, honestly, we have so many Pokemon already, that within a couple more generations of evolutions and new lines, there's not going to be any radically new roles to fill.
There simply won't be a need for any new Pokemon, because any potential role will be filled by an existing Pokemon that already fills it better.