I do not believe that this would actually be the case. Our cars feel heavy, etc., but they are not really that heavy if you compare the actual weight of many modern full-size sedans with the older versions. In addition, despite using lighter sheet-metal, the newer-style structures (especially the main cage/skeleton) of modern cars is likely stronger. There is a YouTube test crash of an old and new car, everyone's seen. Yes, I know that the condition of the old car is questionable and they probably chose an "X" frame on purpose, there. But still, the old boats are not actually as hard as they feel
Moreover, I think that no one will argue that seat belts make the car safer in almost (99.9%) any crash. In fact, they used seat belts in the 40s crash test and stunt vehicles too. Whether or not any particular person needs them installed in their classic is another question. I do not have mine installed, right now, and just drive 45, as an excuse ๐