I guess I’ll throw in my two cents. I’m in the “use each media for what it’s best at and use different media to force my brain to think differently” camp, but I’ll explain why I think that’s the case.
I sketch with ballpoint pen and paper because pen doesn’t smear in my sketchbook, it scans very well (those are the “use each media for what it’s best at”), and it forces me to not be tempted to erase too much by not being able to erase at all (the "forcing your brain to think differently).
Knowing that all of your strokes are permanent makes you think differently about how you are drawing. It compels you to be more accurate, more confident, and to make fewer strokes to get your artwork done. If you are getting better at putting fewer strokes down, fewer strokes means less time, less time means more work. But you need to force yourself to draw that way, and it’s easier to restrict yourself by using a restrictive tool that to discipline yourself in Photoshop where Ctrl-Z is always an option.
I learned to paint with acrylic first, which forced me to get really good at mixing colors because acrylic dries quickly. I learned to paint with oil later which forced me to learn how to paint with as few strokes on top of each other as possible because of the way oil paints work.
Once I got the knack of being able to create a painting with only a few strokes overlapping each other, and getting good at nailing colors on the first try before applying it to canvas, I was able to take that knowledge and recreate the process in Photoshop, which allowed me to create paintings very quickly. I see too many digital artists put down way too many strokes than they need to because they grew up with an infinitely forgiving media: digital media, where the canvas is always wet, always dry, where you can always undo so they draw curves by trial and error, and they spend too much time adjusting the colors after they’ve started painting, or worse they colorizing their black and white underpainting instead of confidentially painting with color earlier on because they didn’t start with a color study. Why? Because unlike painting with canvas, you don’t NEED to have a color study first, and with Photoshop colorizing after the fact is an option (which unfortunately also leads to black shadows which I see way too much in concept art but I won’t go off on that rant).
But yes, do use whichever media is better for the job. I like to brainstorm and sketch in pen, do more serious character design studies in Col-Erase blue pencil because pencil is more accurate than a Cintiq (without having to zoom in, which also takes time), but all my coloring and painting work is done in Photoshop. Unfortunately with Photoshop there’s no one-of-a-kind originals I can sell (which is one thing to consider), but if I don’t want to make extra money off the originals, Photoshop quite simply allows me to put color down so much faster than oils ever allowed me to. And I can work 4 times as fast using the same slim oil painting process in Photoshop.
Can you learn those skills without using restrictive media? In theory, yes, but it will require a crazy amount of willpower not use all of the very powerful (and useful!) crutches available in Photoshop. If you don’t HAVE to rely on it, you’ll work much faster, and the powerful imaging editing tools will always be there when you screw up.
