So for your demand there are 2 solutions, first only allow 1 or 2 distries? with one or two browsers and thats it? thats impossible ^^ but even to force softwareprojekts to use same libs or stuff is not possible, because Linux/Opensource is freedom, so each man can do what he wants at least if he gets not money from a company for it.
But where I maybe go with you is what you call standardisation, I would as programmer call it modularisation. Its a automatic process as a developer (if I earn the title) to try to decouple shit, and to first look at the most used solution, and only if that sucks I search somewhere else.
So when I make a new programm or even when I refactor one, I try to use as much as possible 3rd party modules because my code gets smaler and I profit from the work of others when fixes and new features are implemented in the sub packages.
But I see here and ubuntu did the same gnome ahead, so when kde uses gstreamer and co thats great do it. But its hard to merge the toolkit, gtk and qt are so much differnt, Maybe they would go there and together make a gtk-look+feel not what is at the moment there like you can see in minitube, that looks like a gtk app but is qt, that goes maybe in the right direction but its not totaly at the goal. but maybe gnome devs and gtk devs could use some day that as replacement for gtk, but then there is so much gtk magic that is gone.
So ok sinse 2009 qt is also in all oses under the lgpl so its not long that that happend 3 years are nothing against the 20 years this 2 desktop-libs exist ^^.
So yes its hard if you would have me asked before gtk3.0 I would have maybe said lets go to qt its better for developers, but with the css-js directeion they are going I cant deside so easy what is better, I find it genious that they made gnome-shell themable by css files. its cracy how good and radical this is. They really invented the desktop new, kde didnt do that, I know Kde != qt and gtk != gnome but they made the changes for gnome.
Maybe they make somehow a new thing some day that combines the best parts of it to one project but that will take years to make it as good as qt and gtk are today, I fear. The other alternative is that one of the desktops will be used primary in the future (in the past this was in the last years ubuntu=gnome2) but on the other hand is qt so strong on other oses like windows ^^. So its hard to make here a compromise... but the compatiblity layers are getting better so maybe it does not hurt in the future if I programm in qt or with gtk apps it looks nativ anyway, minitube is a good example, its programmed in qt uses gstreamer and looks like a normal gtk/gnome app ^^.