One of the bigger problems with 1-up-1-down is that at natural barriers, two teams would commonly swap week after week, leaving three teams (the one that went up, and the two that remained where it was) getting steamrolled every other week.
This, in my opinion, is a weak argument. The teams on the boundaries between tiers are there for a reason, so they should expect to win in the lower and lose in the upper. That’s exactly how it SHOULD work. Now if the jump at a single breakpoint is so severe that it ruins things for both tiers, then the problem isn’t with the matchmaking but with the servers themselves.
In my opinion, relegation / promotion is simply so superior to what we have now that I wish we already had it.