Solo albums usually are projects a member does between the regular duties and commitments of their current band. It sometimes announces that a band is having problems and will soon break up or lose members.
If "DIO" was a solo project he would have still continued making Black Sabbath albums along the way... or Rainbow.
He started another band and used his name. Members change.. crap happens. Times change and Heaven & Hell became his "side project" or 2nd band. How many members juggle bands to make a living nowadays?
Same thing goes for Ozzy. He started his own band and used his name.
KISS released 4 solo albums in 1978 then got right back together to record as KISS again... then of course broke apart not much longer.
What about The Alice Cooper Band?
The "cover band" debate is always fun to argue (in a friendly manner) but if a band decides to carry on - more power to them! Name change? Possibly, but if most of the members remain, why not keep it? It's easier to promote a known name that starting from scratch again.
Then there's the Tigertailz & LA Guns situations where there's 2 touring bands using the same name.
Jack Russel's Great White vs. Great White also springs to mind.
Steve Perry was not the first singer in Journey, so I find it amusing that people argue the "cover band" issue over them. A personal friend of mine does this all the time, then looked at me blankly as I reminded him Perry wasn't the first on the mic.
After seeing his blank face I hit him with AC/DC for the win.