I see this argument raised quite often and it's a bit like saying English should have it's own alphabet because latin alphabet wasn't made for whatever amalgamate of languages modern English is. There are cultural factors at play as well, Poland was historically a Catholic country and that came with latin included. Basically: digraphs are prevalent in many languages and Z is as good a letter as any.
Other than not having to use diacritics, what benefits does it present for modern Polish users to switch to an alphabet designed for Old Church Slavonic?