You missed the main point. I would agree with you, if

1. "use fields" is just a single use statement
2. its user interface is stable and independent from its underlying implementation

Unfortunately, this is not the case.

If those were true statement, then you would be right, as the compatibility (when one uses the word "compatibility", always ask the question "what is the level of compatibility?") is fully granted.

However, "use fields" comes with a really clumsy interface, which is tightly coupled with its underlying pseudo hash, which has attracted lots of strong criticism, since its birth.

Logically, whether "use fields" would be there in the future is irrelevent here.

Especially if what you guessed is true that the underlying implementation would change to use lock_keys, then it makes even less sense using fields in NEW development, instead of using lock_keys directly.