Wouldn't it be in these companies' best interest to release free "dev" versions of Solaris/HPUX/AIX/etc? I guess even then the problem is there's even a moat there to begin with. Company would have you fill out various info to get a license whereas you can download Ubuntu/Debian/CentOS without any such restriction.
I also wonder about mainframes and why IBM hasn't come out with some sort of "emulation layer" for X86 machines. Yes mainframes are expensive, but wouldn't you want to do everything you can to get mainframe software that people can learn with into as many hands as possible?