Regression

The meaningful point is that "This UI has regressions and therefore should never have been made the default" is a false claim to a measurable change in quality and an implication that, as a result, the choice was irrational. People should stick to "It breaks my workflow" or "I find this change objectionable for some other reason", which are arguments that can lead to actual discussions about whether the improvements to other people's workflow outweigh that, or whether there would be some way to tweak the behaviour such that a specific usecase becomes practical without compromising any other design decisions. Bringing up "regression" shuts that opportunity off, because we all know that regressions are unarguably bad and need to be fixed immediately. People should say what they mean rather than throw around terms they can't even meaningfully define.