and then the new version came out and those that couldn't or didn't want to get a newer PC had to leave
Which actually had been very few players. The system requirements for AH3 where quite low even for 2017. The decline of player numbers had been a very steady one up to AH3 and was notably slower, if not even halted, after Ah3 was published.
So while AH3 was a disappointment for all who had high hopes in getting AH back to former glory, it seems to have extended this games lifespan considerably.
By the way the decline started around 2009, in late 2015 the numbers had dropped so far that I stopped playing regularly because of the resulting gameplay issues in my timezone.
For some quick numbers: In 2001 the game had like 2000-2500 'scoring' pilots per tour, the first golden age of AH1 saw 5000+ in 2003/2004, I joined at 4000-4500 pilots per tour in late 2005.
AH peak was around 2007-2009 at 6000+ per tour. In late 2015 (when ended my regular subscription) it was down to 2000. Nowadays, its almost stable for two years at slightly over 1000 scoring pilots.