You need to be able see a way back down. That might be the end of the overcast a few miles off tracks, or it might be holes of sufficient size within view. When your last hole is starting to leave view you have little choice but to turn towards it and use that way out. If you don't then you run the risk of being in a situation that you describe. I don't think many pilots who don't have instrument qualifications but do have VFR on top privlidges will continue on in such a situation.
If they did continue on, then their principal escape route has to be to return to an area of known good conditions, but of course that could have changed by the time that they return to it.
DP,
Whilst this may be sensibly conservative, I don't think one has to be always that conservative as a rule. I can remember plenty of scenarios of being over an overcast layer with no ground in sight. An example might be Bournemouth to Biarritz. Say there is bad weather over NW France. Dinard, Rennes, Nantes TAFS all OVC. La Rochelle a bit better. Bordeaux, Biarritz, Toulouse, San Sebastian all CAVOK or FEW or SCT. You could spend an hour with no hole in sight, but be comfortable that you will see the ground somewhere further south to be able to descend. The advantage of the IMCr in this scenario is the comfort one gets from knowing that if you have (say) an engine problem in the OVC area, the least of your worries is the instrument arrival and approach. You have a legitimate emergency and all the skills necessary to handle the IFR work to get down.
That's not to say any OVC layer is fine as long as there some hope of better weather further enroute. You need a convincing body of weather information to give you a reasonable and high standard of assurance you can complete a flight safely in accordance with your privileges. It's more to say that I don't think it can never be acceptable to be VFR on top of a layer.