I don't know if you've noticed but today's generation just ignores ads. I work in schools - the pupils do not see anywhere near as many ads as I did when I was a child. TV ads are dead - they are background noise. We've trained children to ignore all ads in games and online. Streaming services mean that ads have to be forced and - inevitably - the kids find a way to download without ads anyway.
I bet you could hum the tune to several hundreds ads if you went through one of those websites that shows you old ads from your country. The kids today? Probably only the extreme ones.
The more you force ads, the more you force people to ignore them if they can't bypass them. It's counter-productive.
Honestly, I think it's more to do with legacy code. Who has the code to some 10 year old early "smart" TV that ran on a custom chip that's not non-standard and unavailable, and so who's going to do the development and testing to push newer formats, HD, etc. down to that TV's firmware.
In my house alone, I have YouTube apps on several phones, a tablet, a cable box, an older cable box, a DVD player, a Blu-Ray player, a cheap DVB-S box, the original Wii, etc. To update all of those to newer formats, HD quality, etc. may not even be technically possible (which just generates more exceptions and differences in the codebase), plus any licensing, plus the risk of breaking the device, plus liaising with all the manufacturer's (most of whom just won't care as they're not selling that model any more), etc. It's just an enormous upheaval for zero gain.
And it's not just YouTube. BBC iPlayer suffers the same fate - all the above devices have BBC iPlayer apps on them too and some of those no longer work because it would need some cheap Chinese manufacturer to bother to develop, test and push a new firmware for a device they no longer sell (or, even if they do, represents a tiny portion of their sales in only a particular country). Just the risk of bricking something isn't worth the hassle of trying to update it.
We are certainly breeding a throw-away culture of technology because of this, yes, but that's not "enforced" so much as inevitable. A Â£20 DVD player with network connectivity and an iPlayer/YouTube app on it - if the app on that stops working? Who cares?
Just the development time alone to push out even the tiniest of working updates for devices like that is enormous. You might even find that the original development team, or even company, doesn't exist any more. Will consumers notice? Not really. They have ten devices that can do the same and they won't be turning on the DVD player to watch YouTube when they can ChromeCast it from their phone or whatever nowadays.
It's obsolescence but not necessarily deliberate and malicious obsolescence. Just necessity.