The Cubs both don't have a DH, have a total unknown at 1B, and have a lack of LHB power bats in the lineup. There's definitely a scenario where Busch and Mervis end up in the same lineup this year. There's also the possibility that Busch gets hurt or doesn't hit well, and Mervis is backup for that, as is Dom Smith. Smith will probably not be ready for OD either, and if Busch gets hurt and Mervis is dealt who would be the 1B (or DH) vs RHP? The Cubs value depth.
Mervis could very well end up being traded at some point, I just don't see it happening before OD unless they acquire another power LHB who could play 1B, like Bellinger, and even then they would probably prefer to wait until the trade deadline or next offseason to deal him. It just doesn't make a ton of sense to me for them to deal from 2 positions that's already one of the weakest on the team (1B and DH). So far the prospects Jed has dealt has been from positions of strength and overabundance, like lefty SP. By next offseason we'll see how Busch handles MLB pitching and should have more prospects who can potentially start at DH on Opening Day like Cassie (LHB), McGeary, Murray (switch hitter) etc.