I personally think that Monero is more likely to outlive Ethereum, then other way around.
Monero has a pretty time-less value, in my opinion.
Not just for privacy, but for it's constant GPU mining as well, due to it's occurring forks to keep ASICs away.
Ethereum on the other hand, I don't know. It seems to be in that very competitive, very fast evolving market. Unlike most altcoins, it is useful, but I do have my serious doubts about it's stability and longevity over decades.