I would have told them exactly what you said in your first paragraph, plus I would have asked them if they know that I'm an active FT poster. Next I would remind them about how important my business is to them.

If that doesn't work, find someone else with carryon luggage who will agree to remove his/hers from the overhead and check it in the hold so that you could fit yours instead.

First off, I do follow the rules when it comes to size. Weight... well sometimes not, admittedly. 8kg isn't a lot and that's why I don't fly Swiss. If it hasn't got any valuables and is actually suitable to be checked in the hold, then it's fine.

Otherwise I'll have to ask for the airline to accept full liability, which, unsurprisingly, they never do!

How do you handle forced gate-checked bags?
 

You'll have to gate-check that bag. Gate-checking means that the bag is taken from you, tagged and given to baggage handlers, who transfer it to the belly of the plane. You get the bag back when you land.