If this system depends on trusting 2/3 of the keys in the genesis block, why not simply sign all individual transactions with keys in the genesis block, and unlock even higher transaction throughput?
I mean if you gotta trust a centralized authority anyway, ditch the blockchain and make it simple.
What centralized authority? The keys in the genesis block aren't owned by a single actor, but by multiple.
You can keep the validator set static all the time (use the ones from the genesis block), this might be useful for private chains. But Tendermint also allows for dynamic validator sets. This is useful for Proof of Stake, where validators can come and go, and their voting power can change in accordance with changes in their stake deposits.