Supreme Court term limits

Why does it matter to our entire system of government that Anthony Kennedy is 76, or that Ruth Bader Ginsburg is 79 and has a history of cancer?  Whether either is able to continue breathing for the next four years will likely determine the future of abortion, gay marriage, campaign finance and any number of other issues, as another Roberts/Alito Justice replacing Ginsburg or a Kagan/Sotomayor replacing Kennedy would shift the balance of power on the Court.

Why not have a Supreme Court term of something like 10 or 15 years, after which they ride off into the sunset?  The pressure to nominate someone young and healthy prevents older but more-qualified judges from  reaching the Court.  I suppose one could argue that you wouldn't want a tremendous justice's potentially-longer term to be wasted by kicking them out after a decade or so, but the same argument could be made for Presidents, who are put out to pasture after 8.  And it's not (so far as I'm aware, anyway) like the current Justices have such far-and-away better legal minds than other judges.

It's important that Justices not be eligible for appointment to another term, as that would politicize the Court. But taking biology out of the picture by limiting Justices' stay on the bench would make for a better Court.