I think it's a case of the Grail can't cross the seal, but you have to drink from the Grail periodically to keep the immortality up. You couldn't just drink once then toss the cup and live forever in that room. Still functionally the same answer, but a bit more nuanced.
Why would your immortality run out? Seems like it would be a "for life" kinda deal. "Immortality - you live forever...until you don't". How would you know when your immortality is running out? I guess you could do things to test it out, but it seems kinda risky. Suppose one day you test it out, and, whoops, all the juice is gone, and now you're dead. I took the warning as "you're immortal, until you cross the great seal".
Last Crusade, is a really fun movie, until you get to the challenges. Then it's like Lucas/Speilberg stopped paying attention.
For instance, in the first challenge; the penitent man. How does "the penitent man kneels before God", become "the penitent man tucks and rolls before God"? What clue gave Indy the idea that after you duck, you need to do a somersault? Was God holding auditions for Cirque du Soleil: Heaven?
Then, the second challenge; Word of God. When Indy steps on "J", he falls through, and we get an undershot of the walk way. We see there are only a few pillars (presumably under the correct letters). But when Indy is dangling, there are no supporting pillars - he should have fallen to his death. He pulls himself up on what looks like a letter "P" (which I believe is "R", in latin). Considering there are even less Rs in Jehovah, than there are Js, why didn't he fall through?
Admittedly, I really did like the optical illusion of the third challenge. Even though I know it's there, it's still cool to see.
Then the final challenge - picking the grail. If I was Donovan, I'd be like "woman, this is my life we're talking about. Could you maybe spend more than 8 seconds looking through this ancient Pier 1 catalog, before deciding which grail is real?". I get Indy's line of thinking - ***** was a man of simple means, so therefore, he wouldn't have anything fancy. But just because you eat off of paper plates when you're by yourself, doesn't mean you don't break out the fancy china when company is over for an important dinner. For all Indy knows, maybe ***** had a big ol' Snoop Dogg cup, for when he had friends and family over for dinner.
-Fred