As there was no reply from any witness or one of the really big accounts, I messaged some witnesses to hear their opinions about the topic. So far only @timcliff replied (thanks for that, even if I would have appreciated a real discussion under my article even more).

The essence of what he said was the following:

• The idea of diminishing returns had been proposed by a few people in various forms and was being discussed, but there was a lot of resistance against adopting it. (Of course I would be interested here in the reasons for the resistance).

• Granting less curation reward for upvoting articles written by high reputation members is considered to be a punishment for being successful.

• It seems that I was simply wrong thinking that voting power is wasted when upvoting old articles. @timcliff explained me that it has no positive effect on the article, but at the same time also no negative impact on my voting power.