Viability: hmmm, at the moment:
No voice calls (no VoLTE) on Pixel 3A (which WAS a primary supported phone for UT), on UT in USA (which is the third largest phone market by country, or 4th largest by region, after Europe).
[alternative: VOIP - NOT available in openstore for most recent version UT]
That is pretty NON-Viable, if you want a phone you can make calls on.
Is that the fault of the UT teams ? I would say NO, it is the fault of the much larger system cancelling 2G, 3G, (USA & Australia , etc, now, and changing worldwide, with plans in place over 10 years ago) and the fact that the drivers for the radio chips and most others are closed source or Android or whatever.
Who would be the ones to fix it ? UT dev teams, if at all.
Or, what about WiFi calling ? Not functional.
Is that the fault of UT devs, or other main stream code (from the larger base) being unavailable ?
OR some phone brands / models NOT supporting required radio bands in some countries. NOT Viable.
I think PC's are a better analogy than cars, since we are mostly talking software here, not hardware.
if using cars analogy, could mention how air bag manufacturers caused a huge recall problem for a variety of car manufacturers.
Q: So, where did UT come from ?
A: 1) a gigantic base of hardware and software: major players who recently CANCELLED (in some countries, and in process in others) the fundamental ability of UT to make phone calls, through changes to the larger hardware and software system.
A: 2) a few dedicated, volunteer developers who are doing their best to modify code for specific devices to overcome above obstacles.