I suspect there will be some hold-outs, who are still looking to keep old analog equipment working, far into the future. For instance, I still have vinyl records. My cassedt deck quit, just haven't thrown it out yet. I have a friend who still has a working BetaMax player/recorder and a whole collection of Beta movies. AM radio is still around.
Every Cable STB, Satelite RX, VCR, DVD player and Laserdisc player has a modulated NTSC output. Usually Ch 3 or 4, but a lot of newer ones are capable of many channels.
Oh, and then there are security cam systems. Lots of apartment/condos have door cams and the best(cheapest) way to get them to every dwelling is modulated on an NTSC channel.
New technology doesn't really replace old technology, just slowly pushes it out of the way. NTSC modulators must be pretty cheap by now. A 25.00 DVD player has a ch 3 output, as well as Vid/aud and s-vid. I suspect NTSC tuner chips cost only pennies.
So, my guess is about the year 2025 before we see the end of NTSC.