One cannot 'prove' something that simply isn't true.
The truth in this matter is not as obvious as you conclude.
I have personally taught over twenty-one thousand students and have worked for a half a dozen dance studios where I had access to statistical information regarding how many lessons every student took, over what period of time, how many dances they tried, how far they got into each dance, how many times they renewed their programs, how many times and how often they re-oriented their program, how often they changed instructors, how often they tried amateur demonstrations, competitions, semi-pro auditions and how many became professional dancers and/or teachers.
Four of these studios made their reputations as being salsa specialists although they offered other partner dances. One of the other two had a pretty balanced salsa/latino/swing/tango/ballroom mix and the other one had a ballet/jazz/hip-hop/salsa blend.
In total, I had access to the learning history of about ninety-four thousand dancers. (It helps when your main job title is teacher trainer. Every teacher talked to me about all their students).
What became very obvious when analyzing all this data is that the average beginner student learning at least three dances (usually merengue bachata and salsa) did so at a slower rate than the average one dance (usually salsa) beginner student.
But the one-dance student had 10% more chances of dropping out during his first program (Those varied in length between 7 and 12 lessons) than the three dance student and 15 to 20 % more chances of dropping out during the second program. And by the end of the first year of lessons, only 10 to 12% of the original cohort of one dance students were still active and only a third of that number were dancing at a higher proficiency level than the three dance students, of whom, on average 80% of the original cohort was still dancing.
The result? If you are in the rare 3 to 4% of only salsa students who succeed, well congratulations! But about 88% of your fellow beginners are now going bowling or playing the bagpipes to pass their time. You go out dancing to latin clubs that play a blend of music and you have to sit out all the merengues and bachatas so you restrict yourself to salsa-only socials were you mingle with other dancers like you who wonder why the scene is getting stale and there is no new blood coming in.
If hardship is the reason why beginners drop out, we have here a good glimpse into maybe why some experience demoralizing difficulty and other don’t. I am not insinuating that this is the only reason but it is kind of evident…
...My personal opinion is that the way Salsa dancing feels is fundamentally different from Merengue. To me, they are as different as chalk and cheese. This is why it is incorrect to assume that learning Merengue will make someone a better Salsa dancer. For some people it will work, but for others it could put them off.
If we make it our mission to look for differences, well, LA style and NY style are also as different as chalk and cheese, and let’s not even mention casino or timba!
Fortunately, professional teachers make it their job to look for similarities in what they teach, instead of differences. Merengue, Bachata, Cha-cha-cha ARE similar to salsa. They share so many elements that if they were human beings, and we checked their DNA, we could consider them cousins or half-brothers.
As for the fear of confusion or the preference of a “feel” when learning them, it depends entirely on the attitude of the students and their instructor. If we insist on looking for, learning and teaching differences; we certainly will transform an easy, pleasant task into an almost unbearable and complicated one. Salsa may appear complex in a vacuum. But put in the context of its close relation to other Latin dances, it becomes quite easy, especially if we already know some of its elements from another dance. Instead of putting their student’s nose to the grinder when they hit a snag, teachers can simply switch to another dance that helps the student focus on the element to be learned but with an easier rhythm or footwork.