Yes @Pietre, I do agree, several problems will be fixed but one problem will be created: the signatures that you think 'in particular are only required to validate the blockchain state ' will loose their immutability for this simple fact that they are not been hashed. right?
The statement above is false. The signatures are still hashed and committed to by blocks. No immutability is lost. If you don't believe me, please read BIP 141 carefully:
A new wtxid is defined: the double SHA256 of the new serialization with witness data:
...
A new block rule is added which requires a commitment to the wtxid.
...
A witness root hash is calculated with all those wtxid as leaves, in a way similar to the hashMerkleRoot in the block header.
By removing this data from the transaction structure committed to the transaction merkle tree, several problems are fixed:
The data is removed from the transaction merkle tree, and
instead committed to in the witness merkle tree. That witness merkle tree is committed to in the coinbase.