Yeah, ceilings should be easy to establish. The max possible cost of bloat on mobile is (number of mobile users * average phone battery) / average charges per day. The max possible waste on backend will be roughly constrained by the energy use of data centres, and that value is available.
I don't expect the answer to come out very high, relative to other things. But it would be good to see an absolute value, if only to drive home the point that everything your software externalizes to your users has costs multiplied by the number of those users.
And it seems that server usage consumes around 1% of the USA energy budget. Heating, industry and transportation are most of that list. Not sure apps are going to show up.