However, and this isn't meant to belittle you in any way, your questions are often at a very beginner level. And there is nothing wrong with that. There is no such thing as a silly or stupid question. But can you see how it might rankle some riders who have spent decades riding dressage to have someone who didn't know if a bareback pad was legal tack in a show make a pronouncement on what is or isn't "real dressage"?
Dressage tack has been developed over hundreds of years to put the horse and the rider in the best position possible to do the movements. A shoulder in is a shoulder in, you're totally right about that. But riding a "correct" shoulder in is made easier by doing it in tack that was made for riding a shoulder in correctly.
Would you want to tackle a Grand Prix jumping course riding in a western saddle? Wouldn't the horn be a little on the unpleasant side as you stretched forward as your horse took those huge jumps? Would riding a cutting horse in a forward seat English saddle make it easier or harder to stay put and go with the sudden stops and turns that a cutting horse has to make as they mirror the movements of the cow and keep it from returning to the herd?
Tack DOES matter. Peel away assumed status symbols of a big name maker or skill of the rider. Each specialized discipline has tack that makes that discipline easier for horse and rider. Would you rather compete in a diving tournament wearing a bathing suit designed to produce as little drag as possible, or would you be okay with competing in a diving tournament in a fencing suit? Both items of clothing are for a sport, right? So what is the difference? Are the divers who compete in clothing designed for their specific sport being elitist by not wearing a fencing suit? Or are they recognizing the benefits of wearing an item of clothing that is specifically designed for their sport?
Paula, you're a teacher, right? I think you teach college level classes? How would you feel if a freshman student walked into your class and based only on reading the course description in the catalog, told you how to define your course and it's materials? Would your reaction be different if a teaching peer who is an acknowledged expert in your field told you how to define your course and it's materials? Do you see what I am getting at here?
If you play a game of field hockey on a basketball court, are you playing field hockey or basketball? Would changing some of the rules of field hockey to make use of the baskets at either end of the basketball court mean you are now playing "traditional" basketball?
Respect the traditions of each discipline you dip into. There are hundreds of years worth in just about every one. Respecting those traditions doesn't mean you have been co-opted by the ruling class. You can still be a rebel (like my instructor who competes in a shadbelly that has hot pink lining and points).