Bullet technology has gotten better with time. This is true. However, what we consider top of the line today could be deemed insufficient as compared to what is top of the line in 20 years.
With any industry, as technology progresses, the results of those products tend to get better. The best navigation systems found in cars today, will look silly 20 years from now.
There is one factor we have not discussed. Human tissue (skin, bone, muscle...), will be exactly the same 20 years from now. That will not progress like technology does. A bullet capable of penetrating a threat 20 years ago will perform exactly the same today.
Shot placement will always be important, so practice up!! Show me self defense ammo from 50 years ago, and I will show you ammo that will still kill you today!!
-George