That's because you can't see it
As happened with Netflix, more and more videos/channels are starting do 4K. Some even have HDR - and it's be great to have everything available one device. Regarding the new device, my hope would have been that they'd fix that YouTube issue and be friends once again :/
That aside, I don't really have an issue with the current, barring the price tag for a really old unit
HDR would be the only exclusion, pure resolution means nothing.
The odd video that I spot as having 4K I'll flip over to the WebOS version of Youtube and to date I haven't found any of them being HDR and therefore haven't missed anything.
Youtube is still just "crowd sourced" content at the end of the day so I don't have the same expectation of it that I do of Netflix and Apple TV+ etc where everything should be Dolby Vision or GTFO.
As for the "really old unit" have you considered how old the unit is inside even the newest of TV's? They don't really need anything more. It's not something that requires feature updates or more processing for ultimately still doing the very same thing.
It's only two years old anyway and running the equivalent of iPad Pro in there so it really doesn't need much.
In fact, my original non-4K Apple TV is still in use in my other lounge and obviously except for the 4K factor they operate identically.
There really should be no concern for how old it is until the standards change, which will only be after HDMI 2.1 becomes commonplace, which anyway means it will only matter once you get a new TV that supports that.
If it helps ease your mind...
1st Gen - 2007 (beginning of)
2nd Gen - 2010 (end of)
3rd Gen - 2012
4th Gen - 2015
5th Gen - 2017 (end of)
And a reminder that the 4th and 5th generation are exactly the same and really one and the same product with an output difference. The 4th gen has not been obsoleted and is still being sold alongside the 5th gen.
I honestly don't think we'll see a replacement before at the very earliest 2021, but likely even well after that.
*****
All of that being said since you don't like iPhones and don't do any other Apple things that I can recall I don't understand why you would want this over say a Xaiomi Mii Box or the above mentioned Amazon Fire options for less money and more compatability with your glorious Huawei.
The bigger benefit of the Apple TV is it's cohesion with other Apple products, without that it likely doesn't make much sense for a non-Apple person to use.
Why do you need a streamer though? What's wrong with your TV's built-in functions?