I don't think we argue too much over terms when the term is the defining glue over what H-F is all about. There's nothing nebulous about the term hi-fi and it hasn't changed much in 50 years. As I've said before, hi-fi is that which produces sound with a high amount of fidelity.. That is, if it sounds sufficiently like real life, or at least sufficiently like the crummy recording of real life. Not much equipment meets that criteria. Once a piece of equipment does meet that criteria, it is high fidelity and it will always be high fidelity because the sound of real life will not have changed. It has become common to cnfuse hi-fi with high end/exotic/exclusive. Ultimately the goal of any hi-fer would be the elimination of low fi and mid-fi entirely. Once we get to a point that both the high and low end of equipment are all high fidelity, there would be nothing to argue about. Right now that is not the case, there is plenty of equipment that does not meet acomparison against lifelike reproduction or accuracy to the recording.
Yes, the old flagships are hi-fii. They were hi-fi years ago, they are hi-fi now, they will be hi-fi 10 years from now. M50 was never hi-fi. it will not be hi-fi in the future. Not unless the nature of human hearing in reality evolves to some new design in 10 years will any of that change. Headphones of the 60's may have sucked, but hi-fi speakers of the60's are still hi-fi as well, and more, fortunes are spent on the recovery and restoration of them.
Remember, reality is the governing body not datamining. The new trend toward datamining and accentuated detail could be argued as not hi-fidelity. They are moving toward the surreal of the digital domain rather than accurate fidelity to life.
The exception that only TOTL flagships count as hi-fi and everything else is not is absurd. I presented a conundrum in the HD700 thrread. HD650 is a former glagship. HD60 is a former flagship. HD598 was a former glagship. HD700 costs $1k, is not a flagship, and nver has been a flagship. By the definition of the "mi-fi" crowd, HD700 is $1k mid-fi headphone, while HD650 was formerly a $500 hi-fi headphone. Now doubt the crowd would defend 700 because it's expensive (proving that "hi-fi is a factor of price, not performance), or defend it because it performs well ((indicating that hi-fi is a measure of performance and not a measure of being the flagship.)
(As always I apologize for horrible typos, this computer won't let me edit on this forum for some reason.) But in either case above the cirrcular reference that hi-fi is somehow exclusive of whatever is the best techological performer of the moment and not a measure of all performance audio products doesn't tread water for long. But it does help some people feel good about dumping fortunes to join an exclusive club, even when that club already had the name summit-fi.