No, they will notTo be honest I don't mind that change that much(as long as it stays active as the last played mode and doesn't always switch back to A.I), this way some of the players which usually brought Top Tier Decks into Casual mode might reconsider (one can still hope) if it's obviously marked as training mode instead of ranked progression off.
As it always has been, 99% of players will test and practice new (or even top meta) decks in casual mode, where they don't have any extra pressure and have the chance to steamroll less prepared decks for easy wins.
Last played mode switching back to A.I. after every single match is most annoying though, I agree.