I know you mean good, but exactly this is the problem: Fix known issues with KMail or with KMails backend? - Nope. Write a new E-Mail client which someday, in the far future might work and have all the features we need? Let’s go!
IMHO Evolution had the benefit, that it initially was written by Ximian and brought up to be good enough™. Honestly, I don’t see anyone investing this time, money and energy in a new KDE email client (or in KMail).
I know you mean good, but exactly this is the problem: Fix known issues with KMail or with KMails backend? - Nope. Write a new E-Mail client which someday, in the far future might work and have all the features we need? Let’s go!
IMHO Evolution had the benefit, that it initially was written by Ximian and brought up to be good enough™. Honestly, I don’t see anyone investing this time, money and energy in a new KDE email client (or in KMail).