4. Again, multiple species and the wilderness origin has nothing to do with zones.
but it does have to do with the UI rework and related code bases that allow for multiple growing populations. They will need to rework that aspect of the UI, and we don't know how much effort that will take.
1. List the elements within 4.0 outside of zones that have the assumption of zones. (There are none.)
There are none which we know off. But that does not mean there are none. the only elements we know enough about to say anything about this is bioships, and I hope we all took it on assumption they wouldn't be connected to zones. This remains a valid point because we cannot say 'we don't know if any part requirese zones. their for they dont' require zones.'
3. Multiple growing species and large pop numbers are unrelated to zones. As for the GUI, much of the interface script isn't even on the same planet_view.gui tab, and the work for displaying population growth mechanics has already happened or is happening.
I'd assume most of the code is broken into separate chunks for ease of use, clarity, and scope issues. None of that means that its easy and not time consuming to change every reference to 'manage multiple pop growth program' to 'manage multiple pop growth in old style planet UI program.' Nor does it mean that you can just blindly change program calls and expect it to work right. If they have the time, its possible. But I don't know that they have the time.
Also, something I remembered after writing that post. It was stated in one of the last two dev streams that they've been rolling the beta into the 4.0 branch. So, it's not like they have any of the 'adapt 4.0 to working without zones' done already. They'd basically have to do that work from scratch. At least for the code written for 4.0.
All things considered, I imagine getting zones balanced right will take less time than trying to go back to the old system and do all the rewriting needed to get the old system balanced right. it's about time management now, and whether or not they have the times to balance zones right, I think it would take even more time to undo the zones action.
Finally, I don't know that the benifits are worth it. Both Zones and the Pop numbers changes are going to cause a lot of upset in the player base who don't know its comming until it shows up in the store. That's going to cause a lot of outrage. Right now, it will happen all at once. With your suggestion it will be drawn across a much larger time.
Nothing they do will change that. Every major change stellaris has gone through has caused the same problem. And this won't change that.
Zones aren't that far off from complete either. The biggest change likely to come is the specialization suggestion from earlier in this thread. but that would be giving everyone 3 more building slots and one less zone. The big complaints people have with zones will not be eliminated by that.