Whoa, whoa, wait...
From the point of view of old clients, segwit adds one coinbase output that contains the root hash of the Merkle tree that commits to the witness transaction ids. It uses 47 extra bytes per block, so technically, yes, it "wastes precious blockchain space".
So, 47 bytes per block. That's not too unreasonable. But...
This then gets us to my question that is not being answered. On average, how many bytes in the blockchain will be needed for a standard payment sent via segwit?
Is this ever less than it would be now?
Is this ever the same as it is now?
Is this usually about 50 bytes more per tx?
50 bytes per transaction for fully-validating nodes? This needs to be answered.
It is answered in the BIP:
Transaction ID
A new data structure, witness, is defined. Each transaction will have 2 IDs.
Definition of txid remains unchanged: the double SHA256 of the traditional serialization format:
[nVersion][txins][txouts][nLockTime]
A new wtxid is defined: the double SHA256 of the new serialization with witness data:
[nVersion][marker][flag][txins][txouts][witness][nLockTime]
from the BIP...
the wtxid is based on all of the original, plus marker (1 byte?) flag (1 byte) and witness, which appears to be:
1-byte - OP_RETURN (0x6a)
1-byte - Push the following 36 bytes (0x24)
4-byte - Commitment header (0xaa21a9ed)
32-byte - Commitment hash: Double-SHA256(witness root hash|witness nonce)
all this seems to be above and beyond what would be needed for the normal, plus the nVersion (4 bytes) and nLockTime (4 bytes) are duplicated. To a simple C programmer like me it sure looks like instead of reducing the net amount as required by anything claiming to save space, it is increasing the size by approx 50 bytes.
Maybe its 32 + 4 + 1 + 1 + 4, so 42 bytes?
I am trying to understand enough to implement this, but unless the original tx is reduced by more than the witness data uses, it will cost more per tx.
But dont worry, I was told that it is likely that 100% of nodes will be pruning nodes in the future and all that matters is the size of the utxo. I still await how any new node can bootstrap if all nodes are pruning nodes...
James