What you shoot with depends on what your needs are and what your budget is.
For example, if I have $2500 and want to take pictures of distant birds.... I'm going to grab a 70D and the Tamron 150-600.... If my budget is $25,000 I am going to grab a 1DX, a 600F4, and a 2X teleconverter.
Right, yes, but the point is that FF is clearly better in many more common shooting situations than crop is, which is why the 1-series and the Dx-series are full frame. If pixel density was king for wildlife/sports, M43 would be ruling that realm but it doesn't. Its not even a consideration for anyone even somewhat serious about that kind of shooting. Pro wildlife and sports shooters overwhelmingly prefer full frame cameras and Canon and Nikon both agree that FF is the preferred size for action/sports/wildlife. There is no real technical reason to prefer a crop over a full frame, all other specs being equal, and the only real life reason to go with a crop is cost. If the technical advantage really existed, pros would be putting their $12,000 600mm f/4 IS II's + 1.4x TC's on 7D's all day long and throwing their 1DX's back in the bag. They almost never do, though.
I would go one further.... I would say that technically, FF is clearly better in MOST common situations. I doubt that there is a single person on this forum who would argue otherwise.... the only case, where in technical terms, crop beats FF is for reach limited scenarios under good lighting.
The thing is, there is more than just the technical aspects of sensor size to consider. For the vast majority of people it comes down to what they can afford to spend. It does not matter what the technical specs of a camera are if you can not afford it.
You can't leave glass out of the equation. If I was going out to take bird pictures (tiny birds) and had to pick between a 1DX and the Tamron 150-600 or a T3i with a 600F4, I'd grab the T3i. Too many FF against crop anecdotes involve great glass on the FF and kit lenses on the crop camera.... not a fair comparison but an accurate comparison because if you have the money for a great FF camera, then you usually have L glass hanging off of it, while many with crop cameras do not.
For many people it comes down to size... a SL1 is a heck of a lot easier to carry on a hike than a 1DX, but even here it comes down to personal preference. In my opinion, for going on a week long hike, the best Canon gear you can bring is a 70D, an 17-55, and a 70-200F4. There are better cameras and better lenses.... but they weigh too much!
Of course if I was hiking in the mountains I would quickly change my mind to a 6D with a 24-70F4 and the 70-200F4.... Your subject matter greatly influences the choice of kit too.....
In the end it comes down to what you as an individual want to do and the balance of factors that influences your decision. There is no easy answer.