Why don't just do a SHA256(SHA256(...ALL TXs IN A COMPACT WAY...)) nowadays ?
Even if the premise of your question (that using the Merkle structure is useless) is true, this isn't something that can just be changed.
Changing how block headers commit to transaction data would be a very invasive change, which would be a hard fork, but would also just break literally every piece of Bitcoin software ever written. Unless not changing this means a near-fatal bug for Bitcoin, this isn't even worth discussing.
This isn't even that. Concatenating the hashes instead of using a Merkle tree is maybe a few milliseconds speedup per block, on weak hardware.
I know Thin nodes are doing this
but this is a little bit useless since major branches are required.
I have no idea what this means.