Prices will go up but I think things will remain active as it seems quite a few are staying on 20.
Where you will see the most inflation is in set cards are they will be much harder to find with less pack openings and supply
Are you staying on 20? Are you on PS4?
It will go up. There will be about 85% decline in players on 20 when 21 comes out meaning nothing supplying the AH with new cards to buy or drive any prices down. That's my guess anyways, but I have never honestly been on a prior game after a new release.
My prediction is this...supply and demand. Without a large pool fueling the market (buying packs and posting cards) there will be little supply...so... Up go the prices. I would recommend you buy now or try and make deals with folks who are leaving as those folks will tend to all but give the cards away.
I can see arguments for it going up or down... what do you guys think?