I think the thing is, it seems those against the idea are discounting the 10 years Ted has had with the Mother between meeting her and their conversation in 2024 (not counting the time she'll live past that point). I know I would rather be with the love of my life for only 10 years than spend my whole life searching.
Lily dying would make sense, but that's not any better. What would Marshmallow do without his Lilypad? Plus, we've had 10 seasons to get to know and love Lily, we've only got snippets over one season of the mother (discounting any umbrellas, etc.).
Again, I have to say, I'm not hoping the Mother dies. Knowing Ted has a long life with the woman he loves would make for a great ending. I just don't want them to pull a cheap fake out just to have us think the Mother dies/is dying.