I still believe they can build things in 2-3 years if they are properly motivated. People often overlook that SWGE only took 3 years to build because they knew how much of a revenue driver it would be. If they think Epic will take away attendance, hence reducing revenue, I'm sure they can follow a similar (if not faster) timeline.
The best case scenario is something entirely new by 2026, with a few rethemes spread out. There are a lot of attractions currently that can be taken down and rethemed for minor attendance boosts (Rock & Roller Coaster, Star Tours, Tough to be a Bug, Captain EO returns to Imagination, etc.)
The Dinoland replacement is split into two phases with Indy (depending on how far development is) sometime in 2026 ideally. Shocker, but it's doable.
The biggest challenge WDW (and Universal) will have is if they can convince APs to stick with their APs while Universal offers a whole new park.