I think you might be taking the request too literally. While the OP might actually want to play that old build, the heart of the request is that 5e was programmed in, so they want to try BG3 using the work that was done. That's not as unreasonable a request as it might sound. Plus, I think the EA builds will start creeping in that direction.
The alternative really isn't that different. Its implementing a codebase into a system that is no longer structured to support it. I think there might be a bit of reductionist thinking if people think there is a 5e engine that you can just flip a few switches and the whole thing works differently now. And again, thats supporting and releasing a separate codebase that has the exact same reasoning behind it as to why it wouldn't be the alpha release. Its a different product, its not one they are working on, and at this point have no reason to. Their priorities right now are most likely:
- Make it so everyone can log in
- Fix networking issues for logins that are causing inconsistency
- Make it so people can log in and then have it not crash
- Make it so people can make a character without crashing
- Make it so people can save without crashing
- Make it so people can load without crashing
- Make it so the wide array of video cards that seem to be having issues stop crashing
- Make it so Vulkan stops crashing
- Make it so DX11 stops crashing
- Fix the "cant save while___" hard stops
- Make it so people can leave combat reliably
- Make sure companions dont kill, bang, or leave inappropriately so youre party is weird/ruined
- Make sure quests can actually end and the trees are correct with all the weird bypassing crap people are currently doing
- Tweaking damage
- Tweaking AI behavior
Im sure theres a lot more behind the scenes I can't imagine but core mechanics like jump, push, attack, offhand attacks, and spell specifics are probably being cogitated right now, and there are some people working on it. I look forward to being wrong when next weeks patch 4 comes out and says "Jump is different, companions follow jump, disengage is an action, magic missile no long requires pathing, fighter AOO riposte functioning, gird line battles, 5 foot distance enforced, clear to-hit % indicators..." the list goes on.
Im not saying its bad to want a bunch of stuff, but "Lets go back in time! HUAH!" - I have a high confidence is not their 7am standup meeting team cheer before getting to work and picking up their "the future is yesterday's discarded code" coffee mugs.
Just saying. I also don't know what anyone's actually SEEN of the old stuff and if its as poorly executed as the "good" version, people might be having some bizarrely high hopes on what it actually would be.
My 2 cynical cents