I won't deny that mods extend their longevity. I'm just saying that there are plenty of gamers who are content with the vanilla games. Hell, Skyrim was an immediate success at launch, long before much of its mods came into existence. Same with Fallout 3 and 4. I'd argue the open world sandbox-esque design of the games are the main thing that give them longevity for the majority of players, similar to how tons of people still casually play any of the GTA games (and not just their online mode). Hell, most of the time when I see random social media posts for FO4, it's the vanilla game with people just showing off their crazy settlement builds.