Let's look at the Modern Metagame:
Deck with Blue:
Baral Storm: Will keep playing 4x Sleight of Hand. I think they'll keep playing Serum Visions as well, because Visions gets you all the way to the fourth card if you need it. Might play more than 8 cantrips though.
Death's Shadow: Will keep playing 4x Thought Scour. If Wraith gets banned, they might go to 12 cantrips and play Opt and Serum Visions. Otherwise, I could see them probably replacing Serum Visions with Opt.
UW/Jeskai: Obviously replaces Serum Visions with Opt.
Merfolk: Can't see them taking time off to cantrip, to be honest.
What am I missing?