Something else to consider. Everyone (well not everyone) is complaining about the CRs being too low...blah blah blah. Or how Graz'zt can be beaten by a 20th-level or 24th-level or whatever party.
Now, what are the chances your party is ever gonna catch them alone? Sure, one-on-one (party vs. demon prince) might be a slaughter in the party's favor (and depending on the power level, it should be. There is nothing that can't be killed at some point, if it is statted. Make them 90 HD? Big deal. A 120th-level party (which can be done with the ELH) will kill them. "If you stat it, they will kill it." If you want them unkillable, don't stat them.
But, all smart demon princes and devils will NEVER be caught alone. Caught alone? Gate in/summon demons or devils. Teleport away. Send for help. Whatever.
The biggest thing about "Oh we whipped Orcus' ass in no time flat." Good for you. Now, if your DM played him anything like I've seen and read a lot of other DMs playing demon princes, dragons, etc. then he didn't play him to his fullest potential. He played him as just another monster. And that fault lies with the DM, not the designers.