I already have Dark Souls on PS3 and PC, and Dark Souls 2: Scholar of the First Sin and Dark Souls 3: Gold Edition on PC. So I'm already as multiplatform as I probably need to be, but I just can't pass up the notion of keeping my skills up to sharpness on the go.
To be fair, they probably could have gotten Dark Souls 1 working on the Vita if they really tried -- it's at about that level of polygons vs textures already, so I think the more powerful souped up Tegra X1 SoC in the Switch should be able to handle it with aplomb -- it's a little more powerful than the CPU/GPU combo in the PS3 so in theory, anything you can get to run on the PS3 should be able to run on the Switch with few problems, and coding for it is infinitely easier because ARM architecture like the Tegra X1 uses is far easier to work with than Cell.