I'd like to have a comprehensive explanation why such a lens doesn't make 'sense', whether economically, technically, ergonomically, etc. Please support each reason with some details so I can learn something.
I'd love to have such a useful lens so I don't have to lug around 2 bodies, each with a different lens to cover as much range as I might need.
There are a number of practical reasons that no 10-300 will ever be made. Most "super-zooms" have about a ten to one range (30-300) and they trade image quality and other things to get that much. High end zooms typically have a zoom range of only 3 or 4.
Its hard to get even average image quality as the zoom range goes up. Too many engineering tradeoffs.
With a wider zoom range, you end up with a big lens, because a 300mm lens is going to be about 5 or 6 inches long even if its a very slow lens (F5.6 or worse). If you try to make it a fast lens, say F4 or F2.8, then in addition to being long, it will have a huge diameter. This has several downsides:
1) big lenses are expensive 2) big lenses are clumsy 3) big lenses are hard to hold still
If you want a 10mm lens, or even a 10-24mm lens, you expect it to be small and easily handled.
Since the existing EF 28-300 is nearly $3000, and a 10-300 would have to be more expensive, who would want to spend $4000 or more? No professional that is for sure, and that is way more than a consumer would want to pay.
DLSRs allow changing the lens easily and quickly. You will be a lot happier if you get three lenses, 10-22, 17-55, and 55-300
The main difficulty is that 10mm is a very wide angle lens. I don't think that there even exists a full frame zoom lens that wide for ordinary consumers. (I think that the 12-24 Sigma lens is the widest zoom available.) Making a crop sensor lens that wide is easier, but it's still a very wide angle lens.
Also, the bigger the range is, the more you have to compromise. You simply can't construct a lens that manages that range and produces really low distortion over the entire range. Also, the lens elements get more complicated, which makes the lens more expensive.
A range like 30-300 is more reasonable, but that is still a big range. That kind of lens is called a 'superzoom', and are quite expensive. For example the Canon EF 28-300 lens at around $2700.
A lens with a range like 10-300 would simply be way too expensive for most people. For that kind of money you could easily hire someone to lug around the cameras for you.
Everything is life is a compromise, when you "invest" in one area (like zoom range) you have to give up something is at least another area (for example: image quality, price, weight and/or size)
A lens with such a huge zoom range will probably have below average image quality (simply because the glass structures you need to focus light as 10mm is so different than what you need at 300mm) - so it's a low-end consumer lens.
Also, it will probably be a "slow" (high f number) lens (see this question) and professionals usually don't buy lenses with such a huge range anyway (because they tend to have lower image quality) - so you probably can't sell this lens to professionals.
Now, there are already 18-200 lenses out there, and they are already priced at the top of what a consumer is willing to pay.
So, how much extra will consumers taking pictures primarily of their kids and families will be willing to pay for the extra range (especially if that lens is also very large and heavy)? my guess is - not enough to cover the R&D and production cost of a super complicated lens.
