I think NR2003 will always be a small community compared to other games but retains an everlasting core group of players, painters, and viewers (mod/track creation has been very low for several years though). Its not really ever going to be a thing where NR2003 will 'die' becasue of its graphics. Just look at the 1999 Roller Coaster Tycoon series, its still ultra popular as ever as player have the openrct2 version, that game is literally just sprites but the gameplay is what keeps everyone around, I see NR2003 as the same.
As for user engagement, I get more youtube views for my ICR series vids on my own channel and engagement than expected which is cool. NR2003 is one of those games its fun to play but also watch too. Like the last 365 days on my channel I pretty much have a baseline daily views of 30-90 which again, for some obscure NR2003 dedicated channel isn't that bad. I only post 1 new race video like every week: (lifetime channel since I created it in late 2019 is already at 20k views).
Not everyone wants to race online either, I'm sure I'd make a decent driver that races clean but I have no interest in trying to be competitive and get on at certain times when I play NR2003. Instead I play for a challenge in offline racing while also having fun racing in my own series' of painted cars and see how it all unfolds.
There really just isn't another modern Nascar game that replicates the exact needs some players want that NR2003 offers. iracing is a hardcore cockpit only sim that you have to pay forever and race against jerks and you don't 'own' the game ever. Then the more arcade games still aren't living up to the classic Nascar Thunder 03/04' days and are just as much as a disappointment currently. I still can't believe a game from almost 2 decades ago is still the better option when technology and detail for game creation has never been more amazing and we can't get a racing game quite like NR2003 still. I'm not complaining though. I have the mod tools and resources to self create content for NR2003 and as long as the game runs on modern machines or I can run it through a virtual machine I'll keep enjoying it until the day some better comes (if ever).