Personally, i disagree with that for several reasons :
First, having just one panel at the left side can be interesting, but it can also create a lot of new trouble as the shell compenents haven’t been made for that. It could create graphical issue when they’ll go to the split-popover look for the calendar, for instance.
It would also get a mess for people that install extensions for some more functionnality : Just by adding caffeine and some other extension, I could make the new vertical panel (that have less space than an horizontal panel) way less usable for Application switching. Remember that Ubuntu also add the appindicator extension by default.
And tbh, even if it takes 24 px more, I think that the current “dual panel” situation have the advantage of being way cleaner in that you get one panel for all the “system” stuff, and another one for all the “application management” part.
If D2P is chosen for default Ubuntu, then only one external extension has to be maintained. Actually, doesn’t have to be maintained, except to change view-app-grid-symbolic.svg to an ubuntu logo icon. Then, the only other extension to maintain would be the desktop-icons. So, less trouble
It would be as much work as Dash-to-Dock : as with Dash to Dock, the desktop team would have to fork dash-to-panel to ensure that it show only the supported settings (in order to have less stuff that could break), and that it can’t bypass QA, exactly as they do to dash-to-dock.
So with all that, I personally think that would be quite a lot of trouble to add dash-to-panel in vertical mode to Ubuntu. Not really any “gamebreaking” problems, but it would add more trouble that it would save imho;