What you said has to be weighed up against the level of support and features you get from a proprietary setup which is often greater than the open source stuff. Our site is quite conservative and they don't mind paying for computing 'insurance' so I would say the odds of keeping the HP-UX status quo are probably high but you can never tell as the mainframe we had was in vogue for decades until they wanted to break free from that with cheaper Unix stuff. We shall see.