So this is a big if but if the new Xbox doesn't play used games would that single fact, regardless of anything else including software, keep you from buying that new console? I personally don't buy a ton of used games but, at times, I still do. But that isn't why I find this concept gross. I'm not even that insanely "don't be anti-consumer!" about issues with the game industry or any other industry.
But this fact would be enough to keep me from buying the new Xbox because it is an example of Microsoft inserting themselves forcefully into a scenario in the worst way possible with a dumb solution. Used games are a problem for developers and publishers. But simply cutting off used games from consumers isn't a good way to fix this problem. That is a slap in the face to consumers who will then turn on those developers, publishers, and Microsoft. Instead offer them an alternative. Be creative and dynamic. Don't just lay the hammer down and expect that everyone will fall in line.
So that's why, if this rumor turns out to be true, I wouldn't support Microsoft and their new console regardless of anything else.