Blizzard’s been very stingy with their sales on Diablo III given that it’s a 6 year old game now. It will go on sale again, but I’d be surprised if it drops much lower. Black Friday sales were the first time the Eternal Collection hit $20 on the other two consoles afaik, and those came out mid-2017.
All that to say - if there’s a chance you’ll play it sooner I’d probably buy it now. If not, there’ll be another sale at some point.