When it comes to amps, there seem to be a plethora of folks who like a specific sound. Some will call that sound ’distortion’, others will call it ’neutrality’ and still others will call it ’transparency’. Tube amps have a specific ’sound’, that sound is generally more pleasant to the human ear than some of the typical ss Class D sound that most folks associate with the class. OTOH, there are folks who prefer this very sound that others decry. At the end of the day, I believe it is the ’flavor’ of the amp sound that we choose...and the appeal to not only our pocket book, but also our aesthetics. Most amp manufacturer’s know this...and design accordingly.
Another question is: has the basic circuit of the tube or ss amp design changed that significantly over the years to call it a total re-design? That is a question for the more technically advanced here.