No-one makes high quality slow lenses in common lengths. You can buy long slow glass, but not middle-length or wide slow glass.

Now in long lenses, they make slow ones because even a slow lens in the 300mm+ class costs over $1000 (assuming half-decent quality, I mean) and on the 400mm+ range you are generally looking at $2000+. Yes, it is true that something like a 400/5.6 or an 80-400/3.5-5.6 has some advantages over a 400/2.8 other than price - it is vastly smaller and lighter, and usable for jobs you simply couldn't do with a 400/2.8 and not with a 400/4 either. (Other than a $10,000 400/4 DO II.) But in the main, people buy long slow lenses because they think they can't afford long fast lenses. (Actually, almost anyone can afford long, fast glass. What people really mean when they say "I can't afford a 500/4" is "I'm not willing to give up unimportant things like take-away coffee and new handbags for one.)

But in short glass - say around the 50mm mark, or something like a 24-70), you can have big and heavy and high quality and expensive, or small and light and flimsy and not-so-good quality. That's your lot. Pick one or the other.

What I'd like to see is high-quality slow glass. Mostly, I use shorter lenses for landscapes and general work. Nearly always, that's at f/8 or f/11. I'm paying good money for an f/4 or f/2.8 lens, carrying all that extra weight and bulk around, and using it at f/11. What's the point? Sure, I'd keep my 35/1.4 and my 85/1.8 and my big 24-105/4 for when I want them, but out and about on a walk, I often wish I had something like a 28-75/5.6 or even a 24-85/4.5-8, which could be very small and light - small enough to drop into a shirt pocket - but properly made (L-Series build quality, or its equivalent in other brands) and with top-notch optics. I'd pay good money for such a lens.

I did buy a very cute little Canon 40/2.8 pancake lens the other day. It might well do the trick for me. It cost $100 second-hand (not much more than that new) and the only problem I have with it is that it's so small I keep mistaking it for a 13mm extension tube.

Anyway, I reckon there is a use-case for high-quality slow lenses, especially for hikers who walk into the mountains and want to take really nice landscapes.