I think we're at cross purposes.

From recollection - there's two different scenarios being debated in the thread - I think we may be crossing them.

The difference between the two scenarios is one where passengers know they have self-upgraded and take the benefits of the cabin that they know full well that they are not entitled to - and this is sometimes evidenced when they are invited to move but don't move and refuse to move and/or argue - and the other where there in the judgement of the CC there is a genuine mistake.

I do however find it as close to impossible to accept that the number of people traveling who try this and claim that they are unaware of the multi-cabin layout or are unable not to simply get into the wrong row by say a single row (I've done this) but get into the completely wrong cabin and take a seat at random - simply impossible to accept. So in my view the vast majority in the group of self-upgraders who claim it is an error - is simply incredible. To further make clear - there will be some - but I think much much less than we are actually seeing.

Personally I don't see anything wrong in the CC offering to sell the seat to someone occupying a seat they haven't paid for and at the discretion of CC if the passenger then immiedately admits an error and apologises - being allowed to move. I think the difficulty is in the group that refuse to move or get nasty.

There is also this other group of plausible chancers or the genuinely incapable.. A difficult issue to manage but at the very least a bit of embarrassment doesn't go amiss.