I completely disagree with you here - both myself and my friend group (say 6-7 folks all 30+) tend to frequent SFMM over Disneyland nowadays. Why?
First, we're all thrill seekers, and sometimes a good, intense roller coaster at SFMM scratches an itch that other parks in SoCal cannot. The park genuinely has some world class roller coasters - X2 is simply unparalleled (clone in Japan aside), Twisted Colossus, Tatsu, West Coast Racers. Even Apocalypse is one hell of an underrated woodie.
Second, the value proposition. I've paid $6/month for years for a SFMM membership - pretty much a bank account rounding error - and that gets me free parking, unlimited admission, and F&B discounts. That alone makes me want to visit more than to pay $206 just to get into Disneyland, let alone ride any worthwhile attraction.
Third, proximity. Living in DTLA, sometimes SFMM is only a 30 minute drive up the 5, versus an hour south.
Like others have said, SFMM has a lot of potential and I'm hoping with the SF/CF merger, that the latter organization can bring a lot of it to the surface through better operations and maintenance.