Here is a product I have been using recently to stop the rot. It is for leather book bindings and I have used it successfully on various items.

Here it is shown on a leather machete scabbard on the right hand side so you can see the difference.

You just brush it on and it soaks in and the cellulose toughens it all up. I am not sure you could use it on anything that you wanted to be really flexible as it is more of a preservative but it seems to stop any more deterioration.
Leather red rot
Leather red rot