it looks to be a nice specs piece for a mild Performance street/strip car... why wasn't produced anymore ?

The old cam was really designed to work in a 10:1
440/3400-3600lb car environment.

By the late 70s/early 80s, 8:1 engines with 40 less CI and 4300lb cars were much more common. People were installing the hemi grind into 1973 400s in a 4300lb car, and were then complaining about the cam characteristics. Not enough low end pulling power, poor idle and so on.

Around the same time, the 272/.455 was also developed in order to be a nicer B engine cam, where there is no compression, gear or converter to fall back on.

The 272/.455 is the sleeper of the mp cam lineup.

The 272/455 was also discontinued recently LOL ( 112 centerline, right ? )

I know actually the 474/280 is the REAL substitute for the Hemi Grind, but was wondering about the original "hemi grind"

also wondering why mostly all the performance stuff was removed from MP catalogs, such as pistons. 383/400 got Domed pistons options back in 80s from MP/DC. If you still were offered these pistons, the old grind cams were full suited and no probs or dissapoints.

one thing takes to the other, etc...

Not discontinued. That part number along with many others is in the process of superseding, because the tappet/lifter content is changing for the better. It's going to take some time.