Backward compatibility is, by definition, an all or nothing binary deal. You can't have it otherwise.

[and yes, this is gonna be unpopular in a post-modernist era where everything get constantly redefined and where there is no such thing as "meaning".]

Sorry that your work has made you so frustrated. It sounds stressful. IMO you should consider exiting your current company or area. Judging by your comments, you are pretty jaded (and set in your ways).

I am not interested in discussing extremes as mentioned in two separate sub-threads now but you do sound like you need a break. Good luck, man.