Even in the boom times of the last ten years - the answer is emphatically to save the money and moan.
- American Airlines made a big play of removing seats - adverts showed the seats being removed. Result? No stampede of extra pax, so the seats went back in.
- Friends of mine who could easily have afforded to pay for BA WT+ chose not to. On one trip (LHR~JNB) they got upgraded to WT+ and said afterwards that it would not have been worth the money and they were glad that they did not pay.
Those that are willing to personally pay for extra leg room are already doing so but I doubt that there are more pax eager to do so. Certainly not regularly enough to make a difference. Expect nothing to change!!!