Bullshit, as long as a game manages to attract a modding scene, it will last practically forever. This is most obvious in the most popular strategy game series: the Paradox games, which have always been modder friendly and have many mods being made for them, occasionally going so far that Paradox hires modders to create spinoff games (like with that Magna Mundi fiasco), and the Total War series where the fan community is still trying to make Creative Assembly improve the mod support of their new games, and modders are actively trying to find ways to change more things in the game, analyzing the code. Mods are still made for Rome and Medieval 2, which are games from 2004 and 2006 respectively, and the fanbase is large enough for these to have lots of players.
There are some other games around that have a strong modding scene thanks to easy-to-use mod tools, and thanks to the uniqueness of the games. Thief has a pretty solid fanbase that has been working with DromEd for over a decade, and over the years there were fan missions that surpassed even the original games in level design. And there are still new people coming to the party, releasing their first FM ever, it's not just the old guys who've been at it for years making these things. And the game is from, what was it, 1998? And then there's The Dark Mod, a modification of Doom 3 to recreate Thief gameplay on that more modern engine. Made by a couple of very dedicated modders who started out as DromEders, too. And there are a handful of people making fan missions for that project, too, as it has a very user-friendly level editor. Then there is the Tomb Raider Level Editor for Tomb Raider 4, which has a very active community that releases new level on a monthly basis, also for over a decade now. A lot of these level designers are women because for some reason, a lot of women love the Tomb Raider gameplay, and most of them don't have a very gaming or computer/programming related background. They're just people who really love one game, discovered the game has mod tools, and decided to play around with them. Not experts at computery stuff, just very dedicated fans.
Then there's the two Neverwinter Nights games, with the first one being more popular among modders than the second because the toolkit of the first is much easier to use. Personally, I fucking hate NWN's engine and a lot of the design decisions that went into it, but damnit, that has a FREAKING HUGE modding scene that is still active. Again, for a game that is older than a decade. The modding scene regularly holds content creation contests, there are some very high quality modules out there, some even made by people who just decided "Hey, I wanna make my own little RPG adventure, this modkit looks easy enough!".
Modding is FAR from dead. Once the mod scene for a game picks up and starts running, you can't stop it. It feeds itself, by releasing ever more great mods, inspiring new people to try it out too, by creating and releasing new custom content that everyone can use in their own mods, getting more and more competent with the modtools, with scripting, with the game engine itself, even with the game's code. The problem is getting it started. Once the modding scene gets started, it's there to stay. But first, the game needs to have a dedicated enough fanbase, or the mod tools have to be flexible enough to allow for easy module creation, and then the scene will grow by itself as modders grow more competent and offer easy starter guides for beginners.
