It's not that simple.
First, Epic's market share would have to be massive for it to work this way. Let's say a game can sell 100k copies on Steam at the avg rate of $20 (to factor in discounts, regional pricing, etc). That's 2 mil and the developer gets 1.4 mil. To get the same amount on Epic the developer only needs to sell only 80k, meaning that 80% of the potential customers would have to move between the stores with ease. That's like a lot and the chances of such an aggressive takeover of the market are low. See THQ Nordic's CEO comment that the "absolute majority" of Metro Exodus copies were sold on consoles whereas it was the other way around in the past.
Second, even if you could sell 80k copies on Epic, why wouldn't you want to sell extra 20k on Steam and another 20k on GOG and another 20k in direct sales? Even if dropping exclusivity means that only 50k people buy it on Epic and 50k people will buy it on Steam and GOG, you'd still earn more: 80*20*0.88 = 1.4 mil vs 50*20*0.88 + 50*20*0.7 = 1.58 mil. No matter how you look it's better to sell on 3 viable stores rather than 1. Before you say 'but it's only for a year", it remains to be seen if the market will still treat a year old game as a new release (meaning media coverage, players' reviews, enthusiasm, price, etc).