The design cycle is especially obvious in the case of Airbnb et al.: they all went from having cursive logo fonts to sans-serif. In other words: the only thing that’s new with the current cycle is that it’s a minimal cycle, making a lot of designs converge. I’m sure we’ll see a larger variety of designs with the next cycle, underlying design philosophy permitting.
I wonder if it's to do with fixed-width serif fonts looking "old" due to their association with old text printers? Now sans-serif looks 'new' by comparison. Couple that with the 'clean sleek minimalist' look that all the modern tech companies have popularized and everything's re-converged on another round of samey looks. We're ripe for another new wave any year now. What'll it be this time? Comic sans? sARcAsTiC?
In other words: "what are you paying all these designers for, if you aren't going to have them match the latest trends?"