There is a point, however, where one starts either modifying the road bike into a hybrid by installing super-wide tires (which don't fit on all road bikes), or one just gets a hybrid.

Here is a reference page I found a while ago about recommended weight limits:

Product and component manuals I Canyon Service

Most of the recommended weights for road bike wheels is less than 110 Kilos, or just under 250 lbs, with some suggesting shorter service intervals for heavyweights. It is interesting that the recommendations for MTB wheels isn't much higher, but they should be designed somewhat stronger to absorb more dynamic forces from bounces.

Obviously you can build a custom bike and custom wheels to be a little stronger. And, of course, it all depends on the caution one exercises when riding the bike.