Question 1: Will Epic stop buying exclusives and, if not, to what extent will they keep doing it?
No, they wont, at the very least for another year and likely even beyond that. Considering the glacial speed of their development progress on features and infrastructure upgrades, it's clear that theyre prioritizing exclusives acquisition as their spend focus. Sweeney's not wrong - forcing people to engage with the store by using exclusives is the only way anybody stands even a small chance of putting a dent in Steam's market share. If they stop buying exclusives, thats a sure sign that they've consigned themselves to becoming another boutique third party storefront like Origin or Uplay instead of jockeying for true leadership.
Question 2: Will Steam decide to lower its percentage and, if so, what will happen afterwards?
Steam will not reduce its basic cut. It may offer additional financial incentives for various milestones (like they're already doing now for big selling titles), but the base rate will remain 30%. It will stay that way unless the industry in general lowers its standard cut (console first parties, Apple, Google), which is not going to happen any time soon.
Question 3: Will Epic improve its service and, if so, what will the reaction be?
Epic will definitely improve the consumer facing side of their service eventually. I just dont think theyre going to pull it off in time to meet the conditions for real success (see question 4, below).
Question 4: Will Epic even be around in the long-term?
(Clearly, this refers to EGS. Epic itself will always be around in some form due to Unreal Engine's ubiquity.)
I don't doubt that EGS will be around for the long term, unless Fortnite doesn't prove to be as evergreen as we all expect.
Epic's path to victory is to build up enough consumer mass that third party publishers will release games on their storefront exclusively (even temporarily) without need for an additional moneyhat, because the lower cut and the Unreal Engine subsidy already make it worth the risk. If all they get is a same day release parity with Steam, nobody's going to buy from EGS given the choice because everyone's entrenched. As of right now, they cant even get that release parity without a moneyhat - 2K, THQ Nordic and Ubi are the only big third party pubs on EGS, with all publicly having disclosed taking financial incentives. Where's Bethesda? Where's Activision? Where is any Japanese publisher? Wheres my EGS release of Rage 2, or Sekiro, or Spyro Trilogy, or Devil May Cry 5, or any freaking Final Fantasy? The fact that these pubs arent signing off on even a parity release speaks volumes.
If they can't achieve that critical mass of paying users after a couple of years, theyre going to be shifting into Origin mode to cut their losses.