I mean sure, you could pack four streams into one 6MHz channel. They could even be "HD" insofar as the transmitted stream is 720p or 1080i.
The result would have been MPEG-2 program streams of about 4.5Mbit each instead of about 18-19Mbit (assuming no other subchannels). That would make live sports or anything with significant motion look like absolute dogshit.
You might say that you don't care about that, and that is fine for you. However, in that case, I would respectfully submit that opinion to be far from the center of the bell curve of television-viewing American public of that era.
We could do a lot better with ATSC 3.0. So, there might someday be a second round of broadcast frequency reclamation, just as there was with ATSC 1.0.
If you got into a time machine and tried your plan two decades ago, the conversations would have gone something like this:
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Government: "Hey broadcaster, we need you to throw out and upgrade all your broadcast equipment that's working just fine today."
Broadcaster: "Why?"
Government: "So we can auction off some of the broadcast channel bandwidth."
Broadcaster: "Why else?"
Government: "That's really it."
Broadcaster: "How about instead, I take a little bit of that money you're asking me to spend, and give it to your political opponent's campaign?"
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Government: "Hey citizens, we need you to buy a new TV or buy an adapter for your current TV that's currently working just fine."
Citizen: "Why?"
Government: "So we can auction off some of the broadcast channel bandwidth."
Citizen: "Why else?"
Government: "That's really it."
Citizen: "How about instead, I vote for the opposition candidate who says he'll let me keep my TV and my money?"
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We're able to juggle broadcast frequencies more easily after the transition. Not only did we shrink the broadcast frequency range at the time of transition, we did it again about ten years ago with far less pain.
https://en.wikipedia.org/wiki/2008_United_States_wireless_sp...
https://en.wikipedia.org/wiki/2016_United_States_wireless_sp...
TL;DR: 480i would have been an absolute non-starter.