nemo wrote
Grimly Curmudgeon wrote

Once you've cracked the leather the fibres are all torn
and nothing is going to fix that. There is no cure in a
bottle. Perhaps that's incorrect and unhelpful, but tough.

The same could be said of anything that is broken or split,
yet there are adhesives, glues and filler compounds that work.

Not with cracked leather boot uppers they don't.

You may be right that a perfect cure is unlikely, but
it seems reasonable to me that a fix may be possible,
albeit not necessarily elegant or invisible. Currently trialling
Shoe Goo which is (allegedly) both a glue and a filler.

But does not claim to be useful for cracked leather boot uppers.