In addition to the criteria the OP listed, I always figured a “Megaband” had to have more than one truly gifted musician and/or songwriter in the band. In other words, you’d be able to name more than one dude because each stands out as an accomplished talent all on his own.
So the Eagles would be in, but Wings would be out. Fleetwood Mac would be in but The Police would be out. Genesis would be in but REM would be out.
YMMV.
Lynyrd Skynyrd. Even with half the lineup dead, they’re STILL selling out arenas.
You never seem to hear people talking about Supertramp anymore.
I disagree with this criterion. First, I don’t think the casual music fan probably knows the name of anyone but the frontman/woman in a band. Bill Berry wrote one of R.E.M.'s most famous songs, but most people have no idea who he is. In fact, most of R.E.M.'s writing is collaborative, while Stipe is largely the lyricist.
Second, musos tend to be the ones to determine skill of musicians. I could go on all day about Andy Summers’ reputation around guitar heads, and the fact that he’s played professionally since the 60s, or Stewart Copeland’s influence as a drummer and composer, but I don’t expect the casual fan to know these things. Tony Banks and Mike Rutherford are hardly household names, but well respected… and I consider myself a music maven, but I did not know that Peter Gabriel was in Genesis until after I heard “Sledgehammer” and thought, “Hey, is that Phil Collins?”
In other words, leave R.E.M. and The Police alone! 
You may be right about Wings, especially since there was a revolving door of musicians in the band except for the McCartneys.
Yeah, they definitely fell off the cultural radar when Roger Hodgson left the band. However, about 5 years ago they played Edmonton and they were booked into the biggest stadium in town, and sold about 3/4 of the seats. There were probably 10,000 people there at that concert. So there’s still lots of love for Supertramp out there.
I think it’s the level of popularity that really marks the difference. How many of the bands listed have actually sold out a 50,000 seat venue? REM has been extremely influential and has enjoyed far more longevity than most, but have they ever done that? Maybe it’s silly, but to me there’s a big difference between selling out Madison Square Garden and selling out Wembley or Giants Stadiums.
I agree with Happy Scrappy Hero Pup that the era of the megaband is largely over. Radiohead is about as close as you’re likely to find these days (with the exception of U2, but they’ve been around forever). It’s not simply a matter of “black music” and “white music”. There are far more popular genres nowadays than ever before.
I’d originally included Queen and the Eagles in my mental list, but forgot to write them down.
I would argue that Steely Dan would fit, even though they were decidely not performance musicians (and indeed stopped touring after 1974), but in terms of their influence and continuing popularity, I’d say they’re right there with the best of 'em.
I think Steely Dan fits, and I’m not even a huge fan.
I’m going to nominate a-ha for megaband status. They have had a Number One in the US - “Take On Me,” and the follow up, “The Sun Always Shines on TV” was a Top 20 hit. They’ve always been Morten, Mags, and Pal; they hold the record for the largest ever non-festival paid audience, 200,000 in Buenos Aires; and loads of bands have cited them as an influence: Coldplay, Keane, and even Kanye West!
Not to mention their first album was released in 1985.
(Okay, I’m ready for you all to pick them apart now.)