According to you, the minimum practical length these days is around 70k. (Up from around 50k in the 1960s, judging from the SF paperbacks on my shelves. And that in any series, each novel in the series gets thicker and thicker.)
Has anyone considered reviving the old Ace Double format for slightly-shorter works? Two novellas or short novels (35-55k each) bound together into a single volume?
You'd probably have to abandon putting them back-to-back inverted with two front covers like the original Ace Doubles. (Which could lead to a fight over which of the two appears first.) Ideally, both works in the volume should have some sort of similarity to appeal to the same reader. This could also have potential as a breakthrough medium for newer authors, with the noob's work doubled with a similar but more established author.
It's a matter of what the market will bear. Publishing houses generally do not start major trends; they publish some random thing and the trend sort of springs up on its own. Unlike the fashion industry, the publishing industry is extremely reactionary because it's a safer financial bet.
At the moment, the "novel" of 70-100k seems to be what people want and are buying. The market for short story collections is very small, and understandably so. People generally don't read short stories unless they either seek them out in literary mags or they happen upon them in larger commercial magazines like The New Yorker. A short story collection is something that's tough to read. I know because I read a ton of the old sci-fi stories of the 60s and 70s, the ones that are now in collection form. You read a little story, and then you have to totally switch gears and accept a new reality in another ten pages. While two novellas is not as extreme, the principle is the same - the reader has to switch gears, and readers today don't like doing that for whatever societal reason.
If we saw more short stories being sold and shorter books being sold, we would be more inclined to lower the acceptable word count, but eventually it all comes down to what sells.