I don't have a lot of time at the moment for longer responses, but to think that Genelec hasn't improved or modified their designs in 20 years would be extremely surprising to me. I would also be surprised if there weren't refinements to their waveguides over that time as well. Not dramatic changes, but steady evolution.
There have been many advances in driver technology, our ability to measure drivers, and more importantly our understanding of why a driver performs well or doesn't. Material and adhesive technology is leaps and bounds beyond what it was 20 years ago. Many of the older designs that still compare very well with modern designs are much more so examples of good system design than great components. It's easy to market fancy parts, unfortunately the use of said parts tends to greatly trump the parts themselves.
I wouldn't consider the drivers used in most of the Genelec products to extraordinary. In the 1037 & 1036 they use some extremely expensive and exotic components which have been around for a long time. In many cases we can now compete, equal, or even exceed the performance of such components for much less, and in much less space. In most home theaters we also don't need, nor want speakers with response to 20Hz, but would rather have powerhouses down to 40-60Hz. We are also now looking at designing systems for a wide area of seating and multiple rows, all things not a consideration 20 years ago.
As others have pointed out, marketing drives all, and there is still a delusion that most choices in high end audio are made on solid engineering principals.
There are some very carefully thought out designs with some real skill behind them. There are also many very highly regarded designs in all areas of the market based on some pretty questionable concepts. The fact that Genelec's "old" designs still compete are more a result of solid engineering than the parts they use.
Edit: ... and yes, that was the short