The reality is, however, that a lot of the time one cannot sign off because one cannot get a word in edgeways - short of turning the aircraft around and flying back a bit and trying some more, which is IMHO unreasonable.
Personally I try extra hard to sign off from a radar unit, but it is perhaps less necessary because they can see the squawk change from 0234 or whatever to 7000 so they know you have moved on.
A lot of people give you such a totally casual sort of FIS (Solent comes to mind here) that one doesn't feel guilty about it
In other places e.g. France at weekends (in the past, perhaps not anymore) the service can be so patchy that signing off just doesn't happen a lot of the time even when there is no radio traffic.
I also think that the extent to which ATC will be worried about somebody who vanishes depends on how the person sounded on the radio. If he/she was clearly struggling to manage the flight they might get quite concerned. OTOH if it is some IFR flight OCAS, navigating along IFR waypoints, at FL050, they are less likely to worry. OTOH such a flight is pretty unlikely to be talking to anybody who doesn't offer a radar service - what would be the point??