I'm with you there. Sometimes it's best that a thing have a beginning and end, and just be its own thing.
I don't disagree, but that's not how showbiz has worked over the last couple decades, for better or worse (I think we both agree that it is "generally worse"). Fewer and fewer new ideas, and every big franchise is squeezed out beyond the point of dryness.
I mean, which major franchise that was "finished" with the initial run didn't get worse when revived? I'm not talking about modern reboots of old franchises (e.g. Mission Impossible) or long extended runs (MCU), but revivals/new directions.
The only one that comes to mind as to being consistently pretty successful, or reviving it in an equally good or better way, is Star Trek. One might not like specific series, and certainly there were low points, but it always seems to find itself again - and even the lesser runs (e.g. Enterprise, Star Trek V, etc) had good moments. Contrast this with Star Wars. I know some will eat up anything with lightsabers and the Force, and certainly some later stuff is better than others. But the overall effect--imo, at least--is that of diminishment. Star Wars, as an imaginary creation, seems "less" than it was in 1990, or even 2005. Again, imo.
But yeah. As a general rule, I prefer new ideas, new stories and worlds. This goes for D&D settings, too. For me the "best" version of the Realms was Ed Greenwood's grey box. There was lots of good stuff after, but the Greenwoodian core remains definitive in terms of flavor. Greyhawk even more so.
I think the important factor is that all of these franchises started as one person's imaginative work. Star Wars was George Lucas, Star Trek was Gene Roddenberry, the Realms was Greenwood, Greyhawk was Gygax. All of them continued, and some later creators either did a good job creating content that was "in the spirit of" or altered it in a pleasing way. But some just diverged too much, or didn't get the essential and primary imaginative creation and/or weren't able to actualize it in a way that carried that spirit forward, and even ended up being a mockery of it (see, "Rings of Power").