Why gmod becomed unpopular?

Well yeah, the age has a lot to do with how it's popularity has declined. I also imagine the fact that we're entering a generation where kids/teens/young adults have better things to do than play an old HL2-engine powered game. I feel like a big part of the problem is that GMod has been poorly optimized for so long, it's just a question of "how long until every long-time player gives up in frustration?" at this point.