I’m not sure I agree with the notion that the manufacturers drive the fashion/agenda.
I’ve been riding bikes and therefore going to bike meets for 40 years and it has always been the case that guys stand around talking about upgrades, whether that’s in performance or other. There’s also been the biggest crowd gathered round the newest bike with the latest developments.

Humans have always been drawn towards progression - even in the dark ages there were advancements.

Any company, no matter what they do, needs to make a profit and one of the best ways to do that is: pile them high and sell them cheap.
I’m pretty sure that Honda makes more money in the 2nd world from small bikes than it does from big bikes in the 1st world. If that’s true then Honda would be pushing small bikes in the 1st world….but, up until now, there has only been a small market for them so big bikes it is. I’m not including scooters in this but the high sales of them goes someway towards enhancing my point.

I believe it was the, unplanned, success of the 411 Himalayan that is driving the market for smaller bikes. The Himmie was designed for the Indian market but took off around the world and rightly so. So this penchant is being driven by the market and not the manufacturers.

Most fashions: music, clothing, food etc come up from the streets and are taken up and watered down by corporations for the mass public - I believe motorcycles are the same.
People were customising bikes long before manufacturers were producing cruisers. They were tuning and racing bikes long before as well…… and they were travelling round the world on standard bikes long before the Adventure Bike came along.