Yeah some of the app devs are going to implement support for lemmy while also continuing to support reddit.
Looking at this from a business perspective, I think that would be the smartest move. When a business partner unilaterally jacks up the price, while it might feel satisfying to tell them to go fuck themselves, it's probably not the smartest move. The smart move is to continue doing business while making partnerships with others just in case the asshole partner jacks up the price again.
I think it might be a thing where these apps will use lemmy for the free version while offering reddit for a subscription. If this is the case, we'll see a gradual increase of users on lemmy with a gradual decline of users on reddit. Older people who stubbornly stick to reddit will pay the subscription, while younger people that can't afford a subscription to reddit (and have no attachment to it anyway) will use the free versions and use lemmy.