Top tier must be really large clients for with VERY specific usage requirements. AIX is definitely not mainstream (SMB mainstream I mean) and is usually used for monster mainframes.
Pretty cool though you get to work in those. Ive yet to see HP-UX in the wild...
Some of them are very large, but most are standard Enterprise. You're right though that IBM AIX is not really SMB Mainstream. There's the Power 710, but even that runs like 4 grand for a very stripped down system.
But AIX isn't really used for Mainframes. Even IBM i, which may be more what you're thinking of, is not really mainframe. The mainframe OS from IBM is z/OS, but IBM has a lot more AIX and IBM i customers than they have z/OS customers.
AIX is an incredibly stable, well-supported UNIX variant, so in enterprise it still has a lot of followers. AIX at this point supports memory compression, and clustering, not to mention big RAS features such as being able to patch the entire kernel without bringing the system down. In a way, it's become its own very competent UNIX Hypervisor with LPARs, which I think contributes to its sales