It's not in any way a foregone conclusion that Bitcoin will fail. Bitcoin could evolve into it's own replacement.
Perhaps it already has. e.g. Litecoin
LOL, Liteoin is no advancement. It's just a proof of work coin, very similar to Bitcoin with some tweaks in block generation times, etc.
I like the sound of the Proof Of Stake system, now that's a real advancement.
The POS algorithm is exactly that a Sound.
The PPC POS algorithm is still broken from last December. The "fixes" implemented since have not been publicly discussed nor has it been proven to actually be a viable POS algorithm.
But you are right, the SOUND of POS system is nice...it just isn't a reality yet.