Also - I hope you can take this as well-intentioned commentary, but making a statement like "if I were a builder, I'd just build thin tops and have the customer accept the risk" makes it sound like you've never been in the position of putting your name on something and then handing it to a customer. There's already a huge level of risk and trust involved in building and selling a ukulele, even if you don't make it "more risky" by thinning the top way out. We could go down the rabbit hole of customer relationships and builder reputation once your instruments are out in the wild, but I don't think that's where you wanted the discussion to go so maybe we should just leave it at that.
Ultimately, buyers who want loud instruments and have a legitimate reason for wanting them loud are generally already able to work within the confines of a typical ukulele design (i.e. without needing a builder to build a risky instrument). If you're recording or playing in a group ensemble in public, there's going to be some element of amplification involved, so the loudness question is solved that way. I don't want to speak for other builders, but ultimately things like tone, style, and durability probably trump the ultimate pursuit of volume-at-all-costs when making decisions about instrument design.
Or, for people who just really, really want an acousticly-loud ukulele, the other involved variables can just be maxed out, without putting risk into an ultra thin top. For instance - make the body volume larger, which will not just increase volume but will shift the resonant frequency lower (which, for a uke-sized instrument, gives the perception of more loudness most of the time). This can be done subtly, by changing the curves on the bouts and/or the body depth, to give more volume but generally stay within the confines of a certain instrument shape. Or, add a sound port on the upper bout that faces the player's head, so they hear more of the sound, even if there isn't more sound generated. Also, focus tonewood selections on woods that have better stiffness to weight ratios, which will naturally produce more volume (the string's energy has less mass to displace, and the mass will be more efficient at pumping air). And so on, and so on. None of those things inherently carry the risk of a too-thin top, but all of them can add up to a noticeable change in the perceived volume.
So, ultimately, I hope I'm just reinforcing the point that the whole is greater than the sum of the parts. It can certainly be very fascinating to learn about design and what variables influence what (I think that's probably a major reason why most of the people in this subforum started building). But it's important to not get too hung up on extrapolating bits and pieces of knowledge to the process of buying or building or marketing a ukulele. Simply put, if you want a louder ukulele, just look for a louder ukulele. Don't get hung up on looking for one with a really thin top, expecting that to be the way you get your loudness. Or - if you are going to get a builder to make you a ukulele, don't ask for a thin top. Instead, ask for a loud ukulele, and then have a discussion based on what the builder tells you he is able to do (or not do).