Ethereum has so much network effect that these other projects need to actually pay people to use them over Ethereum. Not to mention they're also more centralized.
A lot of other platform projects are providing bridges which makes it possible to use those projects as some sort of an L2 for ethereum. If a lot of dapps starts migrating to those instead of to native L2 solutions it can get ugly.
However, the actual technical research paper with rigorous mathematical proofs is still going through peer review, but if you email me I am happy to share it with you.
Really, what you need is a distributed hash table (like MaidSAFE), or randomness beacons (like Algorand) to select a subset of the network. This paper shows mathematically that the probability of a double-spend goes down exponentially with the number of notaries, and having the entire network secure every transaction, no matter the size, is wayyyy overkill:
That said currently ethereum has many L2 solutions (https://ethereum.org/en/developers/docs/layer-2-scaling/) that "run in parallel" and result in low gas fees.
So far I've not seen any hint of an "embarrasingly parallel architecture" that can satisfy the safety requirements that a blockchain also does and run smart contracts. I'd be happy to read up if you can point me to any research or projects that I've missed.