Safety is always a difficult sell but as 'the professional party' in the discussion I feel I have a duty at least to try. Joe Punter doesn't know why an RCD/RCBO is safer, no reason why he should, he's just heard the term associated with electrical safety but can't make the mental connection past "what's in it for me?"
So I take a bit of time to tell 'em. I give them a quick walk through of the principle of operation in dead simple laymans terms along the lines of....
The medically accepted upper limit of survivability when a sustained (a few seconds qualifies as sustained) electric current passes through the cardiac muscle is around 50mA which is about a tenth of the current drawn by that 100W light bulb above you. That's the point at which the survival graph starts to tip sharply downwards OK?
If you get hold of a live conductor the smallest fuse in your consumer unit won't even think about tripping until 10 amps or more is flowing through you. If you get thrown clear then you are lucky, if not then you are there until found. The fuse is there to protect the wiring and accessories NOT YOU or your nipper sticking bits of his trashed toy cars into sockets! (optional)
then a bit of explanatory...
An RCD looks at the current flowing down the live and returning up the neutral, if those are different by anything over 30mA and immediately trips the supply to that wire you got hold of and it will do it within 40mS.
Then chuck in a bit about low level Earth faults which normal fuses or breakers simply cannot 'see' but nevetheless are a cause of fire and you can see their attitude beginning to change. The 50 thousandths! of an amp explanation always comes as a surprise to them but is nevertheless true and can't be ignored.
If they are intent on a move in the near future then of course the 'upgrading = sellability' is a natural incentive.