Simpsons never really was a kid's show, though it was popular when I was in middle school... but that was mostly because it was new. I remember a rule that we couldn't wear the "Underachiever and Proud" shirts that were so popular back then (it was the late 80s/early 90s, like you could even see our shirts with all of the glam rock hair), but actually never watched an episode back then, as we didn't have Fox where I lived (since Fox was the new, fledgeling network back then).
I started watching it steadily a few years later, in High School, but the thing back then is that this was the only animated show on in Prime Time. There wasn't Family Guy, Futurama hadn't been cancelled three times, no Archer, no Cartoon Network, so the popularity was a lot more widespread. It was just pervasive, and that's probably why so many of the early episodes resonated so clearly, because it lampooned popular culture and then was popular culture.
All of that being said, I don't know of any kids that watch it, certainly none in Lego's target market. Obviously, my personal opinion isn't fact, but yeah, based on the ratings slice, The Simpsons is basically just my generation's version of Dallas or Gunsmoke... the kind of thing that lasted forever but will end up being what I make my grandkids watch later in life.
The license could sort-of work if they aimed it at adults, but it'd take Lego doing things that I'm not sure they're willing to really do. One is take them out of the toy aisle, deal with boutique dealers and smaller shops, and make a set more mature. This set tells me that they're not ready to do it, given that it's so... Lego. The clamshell design, the system scale of the car, the modern-era-Simpsons lack of humor... these are things that go into kid-focused sets.
Nostalgia can do a whole lot of things to increase sales for adults, look at stuff like the Ewok Village or the UCS line, but I just don't see it working here.