The slowing down of rewards leads me to believe the devs are worried their game will get stale too quickly because once the collection is done, people will churn. (Ironically, I believe the slowing down of rewards will make people churn much faster.) So the obvious and well-established answer is to add another non-gameplay level of collection like foil cards or foil PWs or something.
With another level of truly difficult collection, they could reinstate a decent collection rate on gameplay cards but still have reasons for people to keep playing and keep earning for the hard-to-get “cosmetic” rewards. It adds end-game for minimal effort (no new cards or other IP) and follows well-trodden, working precedent. It allows more people to actually get INTO the game with decent collections (because the devs don’t have to be so horrid with the normal card rates) AND also rewards those who play a lot by giving them bragging-rights bonuses while keeping them engaged. It’s a win-win.