"That would be like working hard for a whole day and sharing the pay with thousands of other people..."

I would prefer to share a huge cake with others instead to have a very small cake for myself alone.

Sure, but the arrival of the huge cake depends on a large number of people acting in certain ways and not in certain other ways. People being people, some of them will be less altruistic than others. How will you get them to act in the name of the common good?

"You can't expect most people sacrifice their time and effort mostly for the benefit of others like that."

Again, if their behaviour contributed to a higher STEEM price, and if they have a lot of STEEM, I couldn't see a sacrifice at all.

People are not ants. From a selfish perspective, the ideal situation is that everyone else does what is best for the common good while you selfishly maximise. That way you get the largest possible slice of the (nearly) largest possible pie.

"As I said, I consider myself as investor, as well, I have earned quite some STEEM within the last one and a half year ... and the value of my account has decreased a lot at the same time: the size of the cake matters, not one's percentage of the cake."

The problem of selfish maximisers freeriding while others put in the effort does not go away by denying it exists. Ignoring it is stupidity.

"As long as a form of PoB is practiced with STEEM, SBD and Vests, it would be in the best interests of Steemit, Inc as the largest stakeholder to help the community fight abusive maximisers and non-economic abusers who harass community members for their personal gratification."

I plead for a committee of elected users with some delegated Steem power from Steemit, Inc., which could decide which stuff to flag and also (in case someone complains) if flags are justified or not, and if "yes" just counter them with upvotes.

I've suggested a similar account before and @tarazkp has done so before myself. The problem with that approach is that it is not a simple thing to accomplish. What I would be willing to hope such a system could achieve is simply to curb the most blatant forms of economic abuse and harassment by whales and large orcas.

In addition, accounts who repeatedly misuse flags in an abusive way (instead using them against spam, plagiarism etc.) could be flagged, as well, after a decision of that committee.

Yes, I think we need something like that. Just paying people for flagging without proper oversight is clearly worse than an exercise in futility.