What rock band is a super group that's managed to hold onto its popularity through the decades and changing musical styles?

ZZ Top

Same three dudes in the “Little Old Band from Texas.”

Formed in 1969, and more than a half century later, they’ll still sell out a show anywhere.

styx. no more to add - 

Mr. Bungle (Experimental Rock): 5-person 'Supergroup' formed in '90, latest album was 10/'20.

I'm pretty certain there are none. A “supergroup” is a band made up of people who were already well-known and successful when the band formed. That certainly excludes ZZ Top and The Rolling Stones.

I might be a bit out of touch nowadays so I¡m sure there are still “supergroups” around but none that have been popular for decades.

Crosby, Stills, Nash and Young, Cream, Led Zeppelin, and Bad Company .

RUSH for the win.  They weren't a supergroup in the typical sense that they were made up of stars from other groups, they were just awesome and each world class players all on their own.  And yes, they stayed on top of the game for decades.