So I need a little help understanding why on earth the eviolite poses a threat to us ever getting regular cross generation evolutions.

Me and my friend were arguing about this topic on skype. What he basically was arguing was that because of eviolite it creates situations where certain Pokémon can instantly become broken. He was using Porygon 2 as an example and how Porygon-Z made Porygon 2 a force to be reckoned with. So then why not remove eviolite from the game? I was getting really annoyed with the whole thing so I ended the chat to not keep arguing.

But I still had a question and was hoping you guys could help me understand this whole thing. If the eviolite can lend itself to creating overpowered mons why are they now allowing for regional variants to take usage from it? For example with Kantonian Mr. Mime and Galarian Mr. Mime both having the same base stats. So it's okay for the Galarian counterpart to benefit from effects of eviolite but not the Kantonian one? Please help me understand this.