It does this by subtly changing the meaning of the fields so that older clients think they are validating the transaction but they really are not, rather than just knowing that they aren't capable of properly validating that transaction version. Specifying a new version number for the transaction lets you be honest with old clients and you can't possibly make the mistake of misinterpreting the context (SegWit or not) resulting in application of the wrong interpretation to the field.
This is disinformation that you're peddling here. Segwit uses an _explicit_ _intentional_ forward compatibility mechanism. Old nodes _know_ that the segwit transaction is making use of rules they don't understand, which is why they will not relay, mine, or display unconfirmed txn using it in their wallets.
By using a tag based system, FlexTrans allows you to skip fields that are not used in the transaction, simplifying most transactions as you do not have to parse and interpret fields that that are not there.
Go look at the history of ASN1 and XML-- 'tag' based formats, they're freeking disasters in the context of cryptosystems. They result in malleability bugs and all sorts of other disaster cases-- Bitcoin itself has had several vulnerabilities related to ASN1 just being used inside signatures. The extreme complexity of FT is also revealed by the fact that it's authors reference implementation had at least three buffer overflow vulnerabilities...
The design also doesn't actually create the flexibility it claims, since changing the fields requires a hardfork. ... and a hardfork could already have done anything. So there is no reason to have an overly generic structure in advance, it simply leads to vulnerabilities and inefficiency.
This also makes the transactions smaller (and thus cheaper) than SegWit transactions.
Actually, the FT format actually increases the entropy of transactions because the ordering of fields is signature normative by not canonically mandated. This means that it actually takes more data to make an efficient representation of FT transactions than ordinary and segwit transactions.
SegWit works by moving data into the coinbase which really isn't what the coinbase is for.
Segwit doesn't move any data into the coinbase. Perhaps you mean that it includes a *commitment* (not data) in the coinbase transaction (though not in the coinbase itself)? This is specifically an intended use of the coinbase transaction, which it has been used for since ~2011... and it's a use that Satoshi recommended-- in fact.
Kinda funny how you asked a question but then followed up with a mountain of pretty extreme misinformation. It's especially amusing to hear you in one breath attack an idea (fields that nodes explicitly ignore) while in the next extol the same concept. It makes it hard for me to believe that you're really asking this stuff earnestly.