I'd also like to put my hand up for getting the wrong end of the stick here.
Fair play mate. Now get reading, and start asking VM questions. 😄
Economy of scale (sort of), yes VM or any ISP could do 'X' but doing this would cost 'Y' and only 'Z' percent of users would notice anything anyway, and probably doing so would result in about 0 increase in subscribers, it's not worth it!
Annoying, but to be fair VM is a business which exists to make money (as indeed do all businesses), so why spend X if you are going to make less back?
Let's be honest here, what percentage do you think of VM users, use the connection to download mostly cute cat videos? Do they care if it's using D 3.0 or 3.1 or 4.0? can I see my cat videos? If yes, then all is OK! Upstream issues, what's that then?
We all know that DOCSIS has issues with upstream, simply because of historical issue of how it works, but do you really think that the majority of customers care less? - it works (mostly); so fine*
* OK I don't personally subscribe to this viewpoint, but I do know how businesses work!
Actually, you're looking at this all wrong too. It costs nothing, so the business angle is a red herring. In fact it would reduce costs, especially in relation to reduced support overhead. It would also better balance available bandwidth and aggregate it properly, resulting in less urgent need to re-segment and upgrade networks (though not entirely, as over-subscription is its own thing too).
Actually, regular uses do care. They just don't know why (yet). Video conferencing stuttering and dropping off during the pandemic? Bufferbloat. Social media jumping around, stuttering, freezing and dropping video playback to potato quality? Bufferbloat. Stuttering and missing words on your phone's WiFi calling? Bufferbloat. One person using the net in the house, and the rest having their browsing and whatnot turning to treacle? Bufferbloat... You'd cut a huge proportion of complaints and support calls (both on the forum and the phone) by properly implementing active queue management. Most of the issues caused on VM's network are due to bufferbloat. The fact that DOCSIS has traditionally been so heavily asymmetric only compounds this, but again can be fixed (eg with ack-filtering).