Why couldn't the coin live on without a team? Does there always need to be constant development for something as simple as Neblio was?
At least some minimal development is also required to keep the coin usable. I'm for example following a coin which was created in 2014 (Slimcoin/SLM), and saw some code updates in 2019/20, but then got stalled again. It's a relatively close Bitcoin fork but with some block structure changes which make an upgrading with new Bitcoin code not completely trivial.
The problem that emerged now is that it is not longer compatible with modern versions of the Boost library. This will lead to the undesirable consequence that every new user needs to either install an old Boost version (which is not very newbie-friendly) or use a pre-compiled binary, which is of course a security risk.
Such "hiccups" will become increasingly problematic once the coin gets older and older. So at least a minimal development activity is needed to help the coin staying alive. It will probably not die directly but simply "wane away".
If the coin always was decentralized and had a bunch of enthusiasts maintaining it at least minimally (what even small coins can achieve, like NMC or PPC), then it's likely the coin can survive the long term as there is always this "community" feeling, not that you're using a "product" from a "business" like it's the case with premined coins.
One miner using that pool, eh? I just don't know if that's legit or not. The coin does trade on some exchanges like TradeOgre, but it's deader than dead as far as I can see.
Both
Neblio and Bitcash seem not totally dead, Bitcash being a bit closer to die, but the blockchains are still "advancing". They probably aren't solving the double spending problem in a satisfying way anymore as they could be probably easily attacked. Maybe that could be considered a requirement for a coin to be "alive" -- to be (reasonably) safe against attacks?