I can't speak to Quorn, although I'm a big fan of portobello mushrooms. By weight, it has not been my experience that mushrooms are cheaper than ground beef or chicken. But also, to say mushrooms aren't a popular and common part of the modern diet... that's obviously not true.
Seitan is awful. If you're looking that hard for a protein substitute, just eat beans. Jesus Christ.
Tofu is everywhere. Probably the most popular vegetarian option in the country by now. East Asians love it. West Coast liberals love it. But you really need cooking oil to make it work (which is expensive and really requires a full kitchen to use). And, again, not meaningfully cheaper than chicken legs or thighs.
None of these compare to staples like rice, corn, and beans, which regularly go for under $1/lb even in our inflated food markets.
People could, right now, choose the cheaper, healthier, ethical, and environmental option
People regularly revert to staples when the price of food climbs. But that's not an ethical choice, its an economic one.
Fast food also has a huge impact on the prevailing diet. As goes McDs, so goes the nation. And the agg industry has a huge influence over McDs, through their own propaganda campaigns to smear meat alternatives as unappealing.
But even beyond that, impossible/beyond substitutes continue to track at prices comparable to beef, especially when you use industrial preservation (pink slime) in transit and storage.
Demand for red meat increases with the economic wealth of a nation.
Wealth per capita, sure. But we've been consolidating our wealth for the last two decades. And, also, ecological collapse.