Exactly. I don't think my view is narrow though, actually quite the opposite. I think you can learn on anything. The camera you use is only as limiting as you let it be.
Exactly my point of disagreement. Yes, you can learn composition on any camera. I have my 7-year-old practice the rule of thirds on a cheap digital point and shoot.
And you seem uninterested in anything beyond basic composition and exposure.
That may be fine for you, it may be the elements of photography that you personally love the most.
But there is a lot more to photography. And different people, different students of photography, may take interests in different things.
For example, many people love playing with Bokeh. Now, I have an excellent compact camera, the RX100. In many situations, it can rival a dSLR or mirrorless. (in fact, the sensor is the same size as some mirrorless cameras). But if I really want to take a great bokeh shot, I switch to my dSLR.
dSLRs gives you the most freedom to explore different aspects of photography, at the highest quality, in an architecture that is shared over brands.
Now, as I stated several posts ago, the lines are getting blurred. Technically, the Sony SLTs can be considered akin to a mirrorless, but to me it is closer to a dSLR.
Out of curiosity from this thread, I briefly looked at the Lumix G3, a camera that I wasn't specifically knowledgeable about. And found that the basic architecture was similar to a dSLR. PASM dial in the same spot, etc. So certainly not a bad camera to learn on, but at the same time, it can't compete with dSLRs in terms of image quality and other aspects.
Go ahead and dismiss this as a war of specs. And certainly, specs can get overblown. Even an expert isn't going to be able to distinguish between an IQ or 77 and 78.
But ultimately, when the question is "which camera is best" -- It's a disingenuous and dishonest answer to ignore specs and say that "all cameras are the same."
Different cameras have different pros and cons. A dSLR (or mirrorless built very very similarly to a dSLR) has the greatest range for a photography student to practice their craft.
Here is a good article on whether a mirrorless can replace a dSLR:
http://www.stevehuffphoto.com/2012/06/30/can-a-small-mirrorless-camera-replace-a-dlr/
The basic conclusion was that for sports photography, dSLR is still king. For portraits, mirrorless can be very good, but dSLR still has a bit of an advantage. For street photography, mirrorless is superior but primarily due to simply being smaller.
Now certainly, a photo enthusiast can do quite well with a mirrorless system. The advantages for their style of shooting may be more important than the disadvantages.
But for someone who is ready to throw themselves into learning about photography, someone who has graduated beyond the Rules of Thirds basics on their entry level point and shoot, I simply see a dSLR as being the most advantageous learning tool.
Certainly, some of my reasons are purely subjective and personal, and other reasons are objective.