I do agree that these bikes based on their age really are no longer meant for that kind of riding
Hogwash, a bike is a machine and is just as suitable now for the use it was capable of nearly 50 years ago. I regularly ride my K1 quite spirited. Usually on twisties (under 100 MPH), but I've done a few 100-110 MPH stretches where conditions were suitable. It pulls well to about 105-110, then tucking in is required. A previous CB750 I had up to 130 MPH indicated and it was still pulling very strong. A real hoot, but only for a moment. Personally I find it more fun to ride a slow bike fast (good technique) than ride a fast bike slow (reasonable speed for street, stay alive, bike makes up for poor or dangerous technique).
In any event, unless you've got a rusty frame holding water and weakened as such I can't think of anything that has changed to make a CB750 unsuitable for 100 MPH operation. Obviously ensure your machine is in good condition before any spirited riding regardless of speed. Key parts to check are triple tree bearings, swing arm bushings, wheel bearings, tires, forks and shocks. I strongly recommend tapered cone triple tree bearings. Worn out ball bearings will absolutely put you in a tank slapper at high speeds.
There's nothing inherently unsafe about 100 MPH. Keep in mind speed limits in the USA are arbitrarily set low and not based on safety, road design, visibility, etc. In most states speed limits are significantly lower than they were 50-60 years ago, think of the vehicle and suspension design at that point in time. Heck, think of the tires.
Do be aware of your own mortality whether you're pushing it at 30 MPH or at 100 MPH. I've been down twice, once at very high speed (tank slapper) and once at very low speed (high side, hit by car in traffic). A good full face helmet saved my life both times. The low speed high side caused me permanent injuries to my spine that really suck, it was way worse than the high speed slide. Had I been wearing proper gear when I had the high speed get off I wouldn't have even needed skin grafts, or to sit in a tub every day and scrub the meat hanging out of me while I waited for new skin. Buy the best gear you can afford, it's far cheaper than a trip to the ER and far better than permanent injuries.