The only reason a 25mm with small image circle is harder to make is because the elements get in eachother's way because of the short focal length, so one might need some more elements for the same simple task?
And that is a very good reason to be unimpressed by it.. a simple normal prime with small aperture, for a high price, only because of the sensor size.
Equivalent on APS-C would be a 30mm f1.6 lens.That lens, although retro focus by need because of flange distance, is not that expensive at all.
And weight.. How much does that Sigma weight? How much would a 50mm f2.4 weight? The 50mm f1.8 STM weight almost nothing.
How about just carrying a FF camera with 50mm f1.8 for comparison, or a APS-C camera with that Sigma? That makes for a way less contrived case.
By the way, where are all those pro's using MFT?
The reason that a fast 25 mm for MFT is hard to make is because of tolernaces, which have to be much higher than for equivaalent lenses on FF. In addition there is the small size, and having to fit everything together within the space constraints of the smaller size, indeed.
There is no 30 mm F/1.6 lens for APS-C, F/1.4, yes, and that is a 3rd party lens, not OEM. Still not F/1.2. F/1.2 lenses are by definition harder to make, and that is regardless of sensor size to start with. It may be equivalent to a less fast lens, but for design and manufacturing purposes it still is an F/1.2 lens.
Neither is this a contrived case. An F/1.2 lens will allow you to shoot in less illuminated circumstances than an F/1.8 or F/2 lens, regardless of the system, whether the equivalence is the same or not.
I am getting tired of this equivalence rhetoric whenever anybody mentions anything related to MFT. That is only useful when trying to compare apples and pears, and you yourself rarely if ever do this when comparing APS-C with FF - I subject that that is because you own both APS-C and FF cameras. In the end it is what works for you, no need to diss everyhting else. Everything else may work for others for different reasons.
Personally, I am not a fan of APS-C, because I think there is not enough difference between FF and APS-C from a weight perspective, (lens) size perspective, etc, but you don;t see me dissing this format - I'll recommend it even, without bias, to those whom I think might benefit from an APS-C system.
There is a much bigger difference between MFT and FF, size wise and weight wise, than between APS-C and FF, especially with my own lens preferences, and that is what makes it interesting. That DoF is a little more at the same f-stops, that images cannot be enlarged as much, and that there is a little bit more noise is neither here nor there if it is in balance with the goals to be achieved. In short, MFT works fro some people, even if it does not for you. However, that does not give you the license to diss everything non-APS-C and non-FF, especially since I doubt you have any experience with the system as a whole. Personally, I have shot a fair amount with a GF-2, OM-D 10, OM-D 5 MkII and a bunch of lenses for the MFT system, and I can assure you that the results are very satisfying. To me more so than with APS-C. And for everything else I use my FF system.
Essentially one chooses cameras and lenses for different and for specific purposes and/or qualities. The same holds true for lenses. Also, one often has to stop down a FF lens to get optimal results, or get acceptable DoF, instead of, f.e., having a single eye lash in focus. For a cropped portrait at 85 mm F/4 on FF one barely gets everything in focua from nose tip to cheeks; that can be done with MFT with a 42.5 mm at F/2, allowing for a faster shutter speed in the process at the same iso setting. Yes, a bit more noise, but generally not disturbingly so.
As to pro shooters who use MFT for professional purposes, I have met a few, and there are a few National Geographic photographers who shoot MFT as well, including wildlife photographers. Generally they use MFT systems for its compactness, relatively small weight, unobtrusiveness, and more than enough picture quality, not to mention video quality.
In short, I can only commend Olympus (and Panasonic) for bringing out more professionally oriented lenses. It bodes well for the future of MFT.
Kind regards, Wim