I don't see Siegel and Shuster heirs being very smart about this. DC could retaliate and KILL Superman permanently which maybe a good thing because since they could promote some lesser known heroes.
Don't underestimate Superman's worth as a drawing card for DC. Most people don't care about the lesser known heroes (as evidenced by the lack of enthusiasm over Green Lantern, and Thor seemed to disappear off the radar really quickly).
However the point that DC could pull something to make Superman inaccessible forever is a valid one. This is the company that's abandoning 75 years of history by restarting Action Comics and Detective Comics, so they could be capable of anything.
Re: 95 year copyright:
^ And depending on when Disney goes back to Washington to update the copyright laws, it could be even longer than that.
I'm waiting for a company like Disney or a record company to make a case for ad infinitum copyright. Plenty of money has been lost by estates and companies through works going public domain - just look to Europe where all of Elvis' early recordings went public domain a few years back, and I think the Beatles are due to go PD in the next year or two. From a paperwork standpoint it would be much easier to simply say once a product is copyrighted (and in some countries a work is de facto copyrighted the moment it's created, no registration needed) it's under copyright forever.
Alex