So your argument against a flexible format is because it requires a hardfork to change existing fields? Well sure, if you change a fields meaning completely then I'm sure it would be a hard fork, that has no relation whatsoever to the benefits of a flexible format. Flex Trans benefit was it is easy to ADD fields as soft forks and OMIT them if they are not necessary. I mean hell, why don't we switch the www to hard coded binary formats, i mean we can just switch to the new format when we need to! No forward thinking required!
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.