[pessimizer]

I see it contrapositively; that they're now so mainstream that they can't find a plot that offends people anymore that they are not also offended by enough to not produce. It's not that society has somehow become unoffendable, but that the people who produce the Simpsons are either 1) people who were hired because they are extremely successful establishment writers, or 2) leftovers from an earlier era of the show who are far more wealthy and establishment than they were then.

It's not completely fair to say they toned it down (although they absolutely did to some extent), it's more that they fell into a establishment rut and repeated themselves endlessly.

You could effortlessly write a humanistic/heartwarming, funny script every week that would deeply offend the powerful. The network that airs the Simpsons wouldn't air them, though. That's not what they're paying for.