I know of no other camera system that comes close to M43 in available lenses. M43 is an open published set of standards. There is one common lens mount. So any M43 body from Oly or Panny can fully use any M43 lens from Oly, Panny, Tamron, Sigma or other vendor.
1. Nevertheless, currently, there are very few third-party non-Oly / Pana m43 lens.
2. The absolute number of lens for m43 is (still) way below those for Nikon or Canon APS-C mounts, even if you "only" count the most modern lens designed for digital bodies (and not, say, old F-mount manual glass for the Nikon, which are, otherwise, still compatible with even the latest Nikon bodies.)
EDIT: the only really missing lens WRT Fuji X mount is, at least for me, the
Sigma 8-16 mm/F4,5-5,6 DC HSM. Sure, there's the Fuji 10-24 lens, but it's two times more expensive and isn't as wide. Nevertheless, 1. the Sigma doesn't have a m43 mount version either. 2. you can easily use it (without introducing cropping) with a passive adapter on X mount cameras.
Also the M43 body owner can use an adapter ring to use older Four Thirds lens. You can get those older lenses on ebay for cheap money.
Nevertheless, the
quality ones (for example, the excellent Olympus 12-60mm f/2.8-4 ED SWD, which, IQ & brightness-wise, is still not matched by a single native m43 lens - the 12-35 Pana comes close but it's in no way as long) are still
very expensive.
Plus some vendors have made adaptor rings so you can mount Nikon, Canon, and other 35mm lens on a M43 body. Be advised that you may have to do manual focus or manual aperture selection with some of those combos.
1. Most of them can't autofocus / set the aperture. Actually, I only know of the (expensive) Metabones "Smart Adapter" adapters to allow for most automatic functionality and allowing for using the OIS when available. And those are, currently, only available for Canon EF -> Sony E-mount and nothing else, meaning no m43 body support is available. (See
http://www.metabones.com/products/search?search=Smart+Adapter )
2. The Fuji X mount also has exactly the same (passive) adapters - that is, you can put any legacy / other lens on it. In addition, as its flange focal distance is 1.55 mm's shorter than that of the m43 mount (source:
http://en.wikipedia.org/wiki/Flange_focal_distance ), you'll have a much easier time using lenses designed for mounts with even shorter flange distance: C mount, Nikon 1, Canon EF-M, Sony E etc.
Finally, your reasoning has another major flaw: you don't take into account that, while there are few native X mount lenses, they're generally much better, image quality-wise, than at least half(!) of the m43 lenses. For example, WRT kit zoom lens, both the Fuji 16-50 and 18-55 are significantly better (and the latter also brighter) than the 14-42 Oly / Pana kit lens. It's only the old 14-45 Pana and the new, GM1-only 12-32 Pana that delivers as good IQ as those rather inexpensive Fuji zooms. This means Fuji kit lens aren't throwaway lens - while, on the m43 WRT "cheap" kit zooms (not the 14-140 / 12-35, obviously), I could only stick with the above-mentioned old 14-45 and the new 12-32 Pana.
Actually, in addition to the low-light performance, it was mainly the lens lineup and the shorter flange distance (allowing for using many more C mount zoom lenses than with m43) that made me choose the X system instead of m43 or Sony's E mount.
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I want to use video - and Sony seem much better than Fuji for that
Yup, if you need as good video as possible, Fuji needs to be avoided. Not even the X-T1 can deliver as good video quality as, say, the A6000.