Yes, it then becomes impossible for Morvek to free himself. Which is why my players would have the choice to kill off all the heirs as a potential solution to the Morvek problem. Not very paragon though. I need to think up some alternative solutions that offer the same amount of security/finality. Any ideas?
It was my interpretation that the demise of a single Quora Tal firstborn descendant would effectively end the threat of Morvek's return. If this is not the case, oops. If it is, it seems unlikely to me someone in the know wouldn't have already done exactly that. The PCs may be paragons, but how many NPCs through the generations since the original Quora Tal have known about Morvek's binding? If the PCs stumbled upon the information, it stands to reason someone else would have, and it seems incredulous to me that not one of them was willing to save the world at the cost of a single life.
That said, it isn't any less contrived than, say, the Fellowship not riding giant eagles, and we all know how popular that story is. Still, I would consider adjusting the conditions that qualify a descendant for sacrifice.
Anyway, I specifically brought up the spell Sequester for a reason. It can preserve the PC's status as morally sound, and there's no standard way to end it prematurely. Of course, as a DM, it's your prerogative to enable Morvek or his agents to find a workaround.
Regardless, it shouldn't be hard to convince a descendant to accept sequestration for at least a short period (say 1 year) to buy the PCs time to deal with Morvek et al. Seeing as how it's in the interest of saving the world, an even longer period shouldn't be a totally unreasonable request for a good natured person. They could also add a clause that ends the sequestration if Morvek is defeated/destroyed.