Let's look at the possible arguments for deleting the question:
Once the new UI went live, the question itself became meaningless. This is true. However, the discussion the question spawned did not.
Questions can be changed, and they can be changed very easily. All that was needed was for a user to re-edit the question from "What do you think of the interface where going to make?" to "What do you think of the interface we've made?" All discussion would have been perfectly valid for either question, because the interface launched virtually unchanged from the proposed version. Any criticism or praises for the proposed interface would match to the launched version.
Even ignoring that, lacking relevance is not sufficient reason to delete the question. All Stack Exchange sites deal in information. That is our highest priority: to create and preserve useful information. Deletion, at its very foundation, acts against this.
Questions should be deleted because the very existence of the question is actively damaging to the site. Spam, way off-topic garbage, exceedingly low quality questions, etc. Closed and locked, the question would act as a record of prior discussions. Deletion means that the prior discussions are lost.
The purpose of Meta StackOverflow is, among other things, to discuss things about the site. This includes changes being made. The record of the discussion of those changes is important. Deleting them simply because the changes were made does not make sense.
This argument is that the existence of the question causes confusion about where it should be discussed. This argument is baseless. Again, the question can be edited to better explain that it is a record of discussion of the proposed interface. So where is the confusion?
The question would be closed and locked; no changes could be made. It's obviously not the place to talk about the new interface. And if there were any confusion, a few words in the opening of the question could clear it up no problem.
Served its Purpose
The purpose of the question was ostensibly to help the developers polish the new UI. To solicit opinions of the community and then make changes accordingly. Once the new UI went live, that purpose was served.
Fair enough. That's a valid reason to close and lock the question, since adding more information would not serve the purpose of the question. But that doesn't explain deletion. Deleting the question means that the discussion it contains is essentially gone.
That discussion was still relevant and important. Why? Because it showed how much the community was listened to by the developers. It shows which complaints were accepted and which were ignored. It shows how the developers reacted to claims about their new UI.
In short: it shows us how much the Powers That Be respect our opinion of their efforts.
That information continues to serve a purpose. Well, not now that it's been deleted, and therefore the PTB's record of respecting the community's opinion is thus more or less untraceable.
Discussion can Continue Elsewhere
If there is a need for further discussion, appropriate feature requests or discussion questions can be asked. This isn't so much an argument for deletion as an argument for why the deletion isn't that bad.
Sadly, this argument doesn't work either. The discussion cannot continue elsewhere. It can be restarted elsewhere, but that's different from continuing. In a continued discussion, you would have pointers back to earlier parts of the discussion that you could look through. In a restarted discussion, the discussion begins from first principles.
Furthermore, the deletion of the question does not help restart discussion. Having the question there for reference would make it a lot easier to talk about things. You would be able to cite previously given arguments. You would be able to cite other posts. And so forth. And most important of all, after many months have passed, you could look back and see where the discussion went, to inform yourself of how best to restart it.
That is not possible now that the question is deleted. Nowadays, if you want to start talking about it, you'll be covering a lot of old ground again.
Are there any other arguments for deleting the question?