I can't help but feel like

Lowering waveclear, and the previous buffs to baron minions, are both in preparation for tower changes/buffs

If you're planning on buffing or changing towers, then lowering waveclear is a way to counter teams turtling under stronger/tankier towers. Which was Riot's sole reason for leaving towers as weak as they were. Now that waveclear is being lowered across the board, i can see tower changes coming next.

I really like this approach by Riot if this is what they're aiming to do!