So...let's resuscitate this moribund body for a tick or two.
At age 78, having been a gun guy since my preteen years, I've watched a number of "revolutionary" and simply evolutionary cartridges come and splash and disappear into the hazy realm of curiosities. Things have, as one would expect if thinking psychosociologically, mostly drifted back toward previous standards, unless there was a compelling reason for the (supposed) innovation to stay. An example is the 7.62x51mm, able to do 90+% of what the 30/'06 can do with less powder and brass in a shorter action, and being the standard U.S. military round. Same with the 5.56x45mm. Same with the short cartridge variants and calibers, such as the Creedmoor family.
From what I read, in what passes for firearms literature now (and, yeah, some is really good), and hereabouts, the 21 Sharp offers the run of the mill rimfire user virtually nothing that current heeled lead bullets don't cover. I've had the same thoughts as above in this comms chain. I can only imagine that Winchester ammunition and Savage got together and said, "Let's throw this against the wall and see whether it sticks...but don't let's spend a whole lot of cash on it". All the above said, I do see one possibility:
If some ammunition maker--could be, oughta be Winchester, but maybe they don't have the capacity-- 1. decided to make ultraprecise Eley/Lapua level "match quality" cartridges as a whole system, bullet, powder formulation and charge, priming, case, and 2. SPONSORED a subgenre of precision rimfire competition featuring MATCH 21 Sharp, in several of the regions where organized precision rimfire is done, there might be a reason for it to exist. Even better if they could pull in other sponsorship. Folks would then be willing to invest in rebarreling, investigate ballistics, get people such as Bryan Litz interested and involved. I'd really like to see this happening...but I'm inclined to doubt that it will.
Otherwise, what's the point? It's a cartridge that's a logical evolution but 90 years out of its time and place.
dk