Well, to add to your sample size of one inline charging adapter, I have two examples along with the corresponding Apple Pencil.
Neither of my charging adapters, nor Pencils, show any signs of cracks or design flaw. All have been in daily use for many years.
As you observe, Apple does not sell the inline charging adapter as a replacement part. However, there are many third-party examples available from online outlets such as Amazon.
As the inline adapter is totally passive in its operation, use of a third-party replacement should have absolutely no impact upon the longevity of your Apple Pencil - the principal life limiting factor of which is failure of the User to keep the Pencil adequately charged at all times.
It is understood that Apple Support, if asked very nicely, have often been helpful with offering replacement inline charging adapters. It is also often reported within these Communities that Apple Stores have also been helpful in this regard.