What's the point of a DSLR?

To take pictures.

A DSLR is just one type of camera. Like all other cameras, a DSLR is just a tool. Many photos can be taken with pretty much any camera. Others, though, might require specialized abilities that are offered in only certain types of cameras. In some cases the size, weight, and complexity of some DSLRs allow them to capture images not attainable with other cameras. In other instances, other types of cameras can produce equally good images in a smaller, lighter (and possibly cheaper) package.

One application at which DSLRs still excel is in the area of low light photography. The problem with using the signal from the image sensor to compose the shot is that the image sensor must be continuously energized. This produces heat, which in turn can increase image noise. Since noise is measured as a Signal-to-Noise Ratio (SNR), when there is plenty of signal (i.e bright light), the level of the noise floor isn't such an issue. But when the signal is weaker (i.e. very dim light), then every little increase in noise can be significant. Even when doing stacked images to reduce noise, the signal must be greater than the noise floor for a useable image to be produced. And stacking images only works on scenes that remain fairly static. For scenes that change rapidly, a single shot is needed.

Another issue to consider is that with many larger, heavier lenses (required to capture more of that illusive low light or to get superior image quality at longer focal lengths) the difference in size between a typical DLSR and a mirrorless body is far outweighed by the lens. Likewise, the price difference between a top end DSLR and other options is also outweighed by the cost of such a lens! In some cases camera stability is easier when the body has enough mass to balance with the lens. In extreme cases, such as with super telephoto lenses, it is all about supporting the lens and the size/weight of the camera attached to the lens is fairly insignificant.

As to allowing shorter focal lengths, the real issue is allowing wider angles of view. Angle of view is a function of both focal length and sensor size. Lenses for smaller sensors can be designed at shorter focal lengths with less correction needed in the lens (compared to what a larger sensor would require for the same focal length), but the angle of view yielded is the same as a larger sensor combined with a longer focal length that requires less correction than the shorter focal length would for the larger sensor. To get the same amount of image quality and the same angle of view with a smaller sensor requires pretty much the same amount of correction as needed for the focal length required to get the same quality and angle of view with a larger sensor camera.