Only speculations from another user now (me), but since the previous desktop app design even predated the UI redesigns that came with the 2024.12.x release (for the browser extension and the web vault), the idea that the web vault could have gotten the old desktop app design, would have kind of reverted all UI changes since 2024.12.x. I don’t think that was an option…
I would further speculate, that the current desktop app changes itself were meant as a further development of the web vault – and the web vault will also get those “design updates” again in the next months/years. (sidenote: so probably this whole discussion here rescued us from an “unusable” future web vault as well 
)
Since the browser extension and the web vault got their first design updates in (or since) 2024.12.x – and the mobile apps also got their overhaul – I guess it “was just time” for the desktop app… and maybe also, because of the outstanding native passkey support and auto-type features, I think this all lead to that current “complete overhaul” of the desktop app.
Apart from that, I would like to once again express that I share most of the general UI/UX criticism of the current state of the desktop app changes. And I have hope that the feedback now will lead to some adjustments.