Ethereum has had unsustainable parameters before (storage mispricing being exploited during the spam attack). It seems you don’t have any appreciation for the fact that these things have repeatedly been addressed by pro-actively and reactively (when necessitated)
CASPER FFG specifically incentivises full-nodes to operate properly. IE be connected, be on time, be available and be diverse in every relevant metric (OS/implementation/hardware/ISP) or be slashed. This changes the model considerably as fees would now price in the difficulty (or ease) of remaining online.
Ethereum is where it is because demand for crypto-currency transactions (both of the value transfer variety and the smart contract variety) outstripped supply, the rising prices signaled to the market that competitors need to be sought and supported. Even if Ethereum does collapse, this demand will keep growing and it won’t be projects like Bitcoin that pick it up, Cardano, the RChain coop and others have done incredible amounts of effort to peer review and verify their designs to ensure they have the stated properties.