digger, I must respectfully disagree with you. Part of your misconception is that you infer that if a newer machine, in a manufacturers line-up, is less-able than their own yester-year machine, that this means they must be "purposefully making inferior detectors", or that "certainly this can't be true, because certainly they must know more now, than they did 20 yrs. ago".
All of what you're saying fails to take into account that basically, we have hit a point of diminishing returns, in this last 20 yr. period, when it comes to detectors. So whereas the period from ..... say .... 1970 to 1990 saw LIGHTYEARS of evolution, every 3 or 4 years (BFO to TR. TR to TR disc, VLF-all metal to VLF Disc, to VLF disc. with TID, to VLF disc with slower speeds, to VLF disc, with added depth AND slower speed, etc...) etc... But think of it: since the early '90s, there has become much & much lesser phenomenal breakthroughs. You're no longer using a dinasour now, to have a machine that is 5 or 10 yrs. old (which was VERY true in the 1970s and '80s, if you didn't update every few years, your friends were kicking your b*tt). There are limits of technology, where you get to a point, where all they can do, is re-package it with hopefully something with more options, easier menus, added back-lights, blah blah. But essentially, it is true, that there are machines of 20 yrs. ago, that are every bit as competitive (and in some cases better) than what's coming out today. So it is NOT a "given", that anything newer, automatically has to be superior to a company's own yesteryear products.
Also, from a marketing perspective, put yourself in the position of the company's owners and boards of directors: If you don't "roll out" a new machine every few years, the sales will drop. So to stay alive, in this competitive market, you have to put out new ideas (even if nothing but whistles and bells addded), and you have to introduce things that promise to "answer" the competitor, or else your competitor will run away with all the market share. So believe me: machines get introduced, for no other reason than supply and demand, and market economics, and they may in reality, have no more depth, or even be a disappointment for some types hunting. The DFX, for instance, I believe was introduced to answer a market demand (to "answer" the Minelab infiltration of the market), and is not necessarily better than Minelabs Explorers, nor is it necessarily better than a 6000 Di pro.
JMHO.