It's all relative to the first break.
There are many great guitarists, vocalists, and drummers who were in bands, or maybe never in bands who either didn't have the ambition or had the ambition and didn't get the break.
To say that any of U2 would have made it outside of U2 is optimistic at best but we surely don't know. The odds would be far, far stacked against them.
In fact you could say the same about most famous musicians, outside of the freak of nature like Hendrix, most the rest of them are a dime a dozen, either fortunate enough to find the right chemistry, string together a couple of hits and amass a fanbase
or they rode on the coattails of someone with undeniable talent (Ringo, for example).
I think people tend to buy into a mythology about bands, that they are destined to come together and take their music to the masses, maybe it's true in some cases, U2 has a strong case, but mostly it's just a manner of opportunity meeting luck and talent.
Point is, if Edge doesn't get the break early, he probably figures he needs to get a job to pay the bills and never develops his mastery of the delay effect, who is to say he'd ever even consider joining a band again much less hit it big. Same with all the rest.
Bono, had a great voice, a very raw voice, but a great voice, he may have had a chance. But what if he's in a band with a crap guitarist? If he gets to 25 and hasn't hit it big, then he might not ever travel to Africa, might not have ever been as politically inclined as we became. He might have been a full blown religous man.
The window of opportunity is so small, I'd say none of them.