I can see that for eldritch blast but book of shadow and pact blade don’t upgrade every time just at 5thBy making them cantrips they follow the existing rules for magic, spells, and cantrips, without having to redesign and reexplain how they work. That is why I support some Spellcasting Class-specific abilities being spells for that spellcasting class. I think we'll see more of that with the Priests and Experts when they get tweaked.