Sorry for calling you out, but "knowing X for a fact" is a strong statement to make.
I don't see a whole lot of Hydrospace machines around. Maybe that had a bit to do with how they are sold here. Maybe it has to do with the price.
, it probably had a LOT to do with Mo.
But it's probable that any OEM looking to get into the 4-stroke standup market would look at the Hydrospace history in the US and take clues from it. If I were such an OEM, the clue would be to drop the idea.
I don't doubt that there is a fair number of standup enthusiasts looking to get 'legal' machines they can ride in their waters that currently, or in the future, ban 2-stroke machines.
I do doubt that there are enough of them to warrant an OEM fronting the development, manufacturing, and marketing cost to bring such a thing to the US market and make money at it, while being 'affordable', whatever that means.
But again, as an OEM I would take a step back and look at the big picture. What you have is a pretty small market with an uncertain legal future. From the OEM's perspective, there are far bigger fish to be had.