Fair enough. Unfortunately, you can probably expect a further trickle of HD channels becoming (or getting initially added) as IP-only on Comcast and therefore inaccessible on your TiVo. As for when Comcast will completely shut down QAM and only offer their cable TV service via IP, that's difficult to say.
Here's what I've gathered about current and future upgrades to cable networks. Some (most?) cable operators -- including Charter/Spectrum (the nation's 2nd largest behind Comcast) -- are going to do one final round of upgrades to their current DOCSIS 3.1 cable network that will involve a so-called "high-split," i.e. dividing the spectrum in their cable in a way that will dramatically increase upload speeds so that they can better compete with fiber operators. For various reasons, it looks like they will be shutting down QAM video as they implement high-split, shifting the band of spectrum used for QAM over to IP usage, resulting in a fully IP-based network. Charter will begin doing high-split in several markets this year. Their sister operator, CGI (Alaska's largest cable co), just completely shut down QAM video in some areas where they implement high-split this summer. After this imminent round of upgrades in 2022-24, the next major thing will be the jump to DOCSIS 4.0, which will start rolling out in maybe 2025. Just about every cable operator, including Charter, says they'll implement a version of D4.0 called extended spectrum DOCSIS, which will keep upstream and downstream data traffic in separate "lanes" (separate frequency bands), but the total range of frequencies used will be significantly increased, i.e. new traffic lanes will be added in each direction.
Comcast, however, seems to be going their own way. They've indicated that their pre-D4.0 upgrades will be more modest, involving a mid-split, rather than high-split, upgrade. And then when they begin rolling out D4.0, it will be of the full duplex, not extended spectrum, type. Full duplex means that upstream and downstream traffic work on the same frequencies, i.e. there aren't separate lanes for traffic flowing in either direction, it's just all together, which is more technologically ambitious. It's less clear when QAM video might be eliminated on Comcast's network. I don't think it will necessarily happen in tandem with their mid-split upgrades, which have already been done in a few spots here and there. My best guess is that they won't shut down QAM until they implement D4.0, which, as I say, likely won't hit initial markets until 2025.
So my guess is that Comcast cable TV customers will be able to continue using their CableCARD-equipped TiVos for maybe another 3 years, until around 2025. And TiVo owners on Verizon FiOS TV might have that long too, although it wouldn't surprise me if they stopped selling FiOS TV to new customers in '22 or '23 and completely shut it down by the end of '24. But for customers on Charter cable TV, I'd be prepared to being forced off your TiVo next year, in '23.
