The wallet format is one thing, another thing is how secret keys are derived. For which there is a zip but not complete agreement from wallet developers. For instance, Ywallet doesn’t follow it for performance reasons. Transparent addresses are not scanned, etc.
Also, current wallets do not support the full extent of the keys from wallet.dat (and certainly not the sprout keys), therefore even if they understand the zwif format, the end user would not be able to recover their funds.
Many people user ywallet as a recovery tool and in 90% of the time, they have the seed so it’s not a format issue.
A common format is still a good thing to have moving forward, but I wanted to point out that backward compatibility may not be achievable and/or worthwhile. In which case, the work is much easier (tedious but simple).