You don't. Most of WDW was better 25 years ago. EPCOT was obviously light years better, but the Magic Kingdom was better overall as well. Hollywood Studios was very different, but I don't think it was really any worse than it is now. Losing Galaxy's Edge would hurt, but you'd get back the backlot tour with working studios and Disney animators, which is probably a wash overall because that was significantly better than Toy Story Land.
Only major thing missing overall would be Animal Kingdom, which I love, but not as much as original EPCOT.
I don't get the nostalgia for the original EPCOT. I had been there a handful of times pre-1995 (pre-teen years) and then not again until 2009+ (well into adulthood), and I found the latter visits to be far superior.
Soarin'? Compared to the Cabaret and the other thing that either preceded or replaced it? C'mon. Original EPCOT + The Land = Solely the boat ride. Substantial gain.
Mission: Space? I have vivid memories of my childhood, but one of them is not Horizons. I would have remembered
something about it if there was something worth remembering. Substantial gain.
Test Track? Ok, the original version was better than the computer version, but the computer version still stands on its own as a great ride. But like Horizons, I have no childhood memory of World of Motion...Couldn't have been worth remembering. Substantial gain.
The Universe of Energy (even pre-Ellen and BNTSG)...Fancy moving auditorium seating and some dinosaurs. That was nap time. No big loss.
The Wonders of Life...Ok, the loss of Body Wars was a substantial loss.
The Living Seas...Ok, a de-theming from Nemo and a return of the Hydrolators would be welcome. But otherwise, it still largely is what it is.
Journey Into Imagination...Ehh, Figment is still Figment, Honey I Shrunk The Audience was cool, seeing Captain EO again was cool, but I really don't mourn either of their losses. The jumping fountains are still the best part.
Spaceship Earth is still Spaceship Earth.
Everything not around the perimeter of Future World (with the exception of Spaceship Earth) was, and still is, a complete dead (to me) zone and is inconveniently large when crossing.
The World Showcase...Other than the conversion from Maelstrom, I can't think of anything else that isn't substantially the same as it always was.
Summing up the substantial gains/losses:
+3 (Soarin', Mission: Space, Test Track)
-1 (Body Wars)
I'm just not seeing the nostalgia.