For quite a few nations the question is also "why bother". ESA could easily turn the ATV into a manned system (Ariane V is man rated) and there is a low key study to do it. But it is low key and a "spin of" for more useful stuff (ATV and it's re-entry version ARV) that does the mission automatically. The whole ARV project has less than 20percent of ESAs budget and the manned subversion is a part of that.
Actually, Ariane V isn't man-rated. It was built to be man rated (for the Hermes space plane, which I got to work on systems for at a time that I can hardly believe was so long ago as it actually was.) As you note there is a low key study to consider going the last round on actually man-rating it.
There's a higher-key study on a new booster, though, more suited to the market as ESA sees it now. And that will detract from any real funding for further Ariane V development on so narrow a use as manned launches. It's more an "ace in the hole" in case other nations become intractable in their negotiations with ESA.
Converting the ATV is certainly an attractive option, particularly if a commercial market develops for man-rated vehicles as a result of work, or the lack of it, elsewhere. It would still be a lot of work, but it's possible. If that moves forward, that might trigger the additional work on Ariane V, but it may also end up on another launch vehicle. A lot would depend on how closely an ATV-derivative interface would be to the original planned Hermes interface, and the final vehicle's mass.
What is another attractive, and very likely option in the current environment is the possibility of a European service module for the Orion. My own opinion is that this would be a great way of each side trading favors without spending a lot of extra time and money. And using an ATV-derived habitation module with Orion is a short step from there. Win-win.
It would be really hard for the U.S. to come up with the money, and time, to cut Europe out of the deep space program or treat them as a second class partner with such a critical component coming from European suppliers (though not impossible, as history shows), and it would allow Europe to take advantage of what the U.S. has already committed to spending its bucks on (capsule/re-entry vehicle and launch vehicle.)
Of course, there's also the possibility that if Dragon/Falcon Heavy proves itself out the U.S. will just drop Orion/SLS as an unnecessary expense, and any related programs will get dropped, too. But then Dragon will probably be available for hire to Europe once NASA's demand has been filled.
