No they won't lock us out of PCs and servers.... now.
But finding a phone with an open boot loader is a challenge today. Finding one that boots in a well described way amendable to replacing the OS is all but impossible. Same for tablets.
This tablets are where the smart people have decided all development is headed, see GNOME3 and Microsoft's Metro UI in Win8. Once they get people to accept tablets as sealed devices that get replaced instead of upgraded the desktop will accept the new reality. Or at least I'd bet that is the plan.
Up to now they have only been toying with us, because they aren't really trying too hard and because they are still learning themselves. So most popular devices get jailbroken eventually, even the mighty PlayStation 3 fell to the hackers. But give it a few more generations and let more and more functions get integrated into a single die and they will perfect locks that won't get broken before the product is obsolete. And they all dream of that day, if for different reasons some of which are are sometimes at cross purposes.
Microsoft dreams of zero piracy and zero competition. Hollywood dreams of charging for every view of their precious content. Government wants the 100% reliable snooping with no opt out and the ability to mandate... mostly because mandating is what the government DOES. The bankers dream of microtransactions for everything... all perfectly secure to ensure they get their taste. With a secure computing environment everyone wins... except the nominal owner of the machine. Of course even that idea is old, now you just get to use it because selling things instead of making you a continuing revenue stream is just so 20th Century.