Ugh. Junkies are just the worst!
Jaxman, I hope you’ll continue to trust your instincts, while still seeking input from your various (potential) customers.
I personally don’t see any point in rushing for a triple tube light, from the perspective of either a consumer, or maker of flashlights.
As a consumer or a manufacturer, the advantage of the 18650 cell is that it is the most ubiquitous LiIon cell type, and with that comes a variety of specific models that strike different tradeoffs between energy density/capacity, power delivery, longevity, and price. It also means that for any combination of traits, the 18650 will generally be equal to or better than other form factors (ie, for a given price and power delivery, 18650 will have greater energy density).
As a consumer or manufacturer, the advantage of any 18650 flashlight is that it takes advantage of the 18650 ecosystem, and, as 18650 cells improve, lights already owned can reap many of the (incremental) benefits.
As a consumer, the advantage of an 18650 tube light is that it is the smallest, most portable form factor flashlight that uses 18650 cells. Indeed, it is small enough to fit into my pockets. It also offers the option of gaining a more compact, portable form factor by giving up some benefits of the 18650 by using an 18350 cell and shorty tube. A 14500 light can be more compact (my EDC is usually an AA/14500 light) still, but, to date, the cells lag 18650 and the best 18630s. A 16300 is, again, more compact, and again, the cells are less capable.
As a consumer, what advantages does a 21700 tube light have over an 18650 tube light? Slightly larger volume. What’s that slightly larger volume good for? Containing slightly more “guts” and therefore slightly better overall capacity and power deliver. It may offer some future-proofing IF 21700s end up being more popular and more advanced than 18650s in the future, but they aren’t now, may never be, and will probably be a long time before that changes. A 21700 tube light will probably be able to sink and dissipate heat somewhat faster than an 18650, but without doing the math, this seems like it will be a minor improvement.
And what do I give up in exchange? At best, 21700 cells are at parity with 18650s in terms of development, but their volumes are lower, and the range of applications, at present, are much smaller, so there is a smaller variety of cells to choose from.
Most importantly though, I think, I give up portability. A 21700 tube light is larger and less portable than an 18650 tube light.
So, as a manufactuer, why should I rush to release a 21700 tube light, rather than waiting to see how the market develops? Well, right now, there isn’t much/any competition, but that could change before one actually gets a light on the market. The size of the market, and its rate of growth is also uncertain. There are, as recent comments here show, some demand, but for the reasons I’ve given above, there is reason to think it may be slow to grow.
Now, as to drivers, I think your current focus on operating within manufacturer specified currents, managing thermals and providing regulated output is a good one, and shouldn’t be abandoned. Offering a hidden “turbo” mode ( whether within emitter specs, or overdriven) with a time out may be a reasonable addition. It doesn’t require an unregulated FET driver, though.
I think selling your hosts is a good way to accommodate the customers who aren’t satisfied with your stock offerings. For the remainder who aren’t willing or able to build their own light, well, they seem like a tiny segment of your market. They may expect you to accommodate their unique needs, but that doesn’t mean you should.
That’s my 2¢, anyway.
(Note, I was/am in favor of the 21700 update to the Sofirn C8F. In that case, the C8-sized head made compactness less of an issue. At the same time, the triple emitters and the substantial thermal path meant that the C8F was capable of high sustained outputs that made the capacity of 18650 cells seem inadequate. I might have preferred a 26650 update, but I think a 21700 was a reasonable choice and a better fit for the look & feel of the light.)