Not many photographers shoot in temperatures of -20 or below, so for most photographers the ability to shoot in such circumstances is not relevant. I have shot mirrorless in temperatures of -10 with no problem, but I can imagine that some photographers' demands would not be met in very low temperatures. For most, these are not really an issue; if a mirrorless camera is better for normal use, the fact that it would be less good at these very low temperatures would be unlikely to affect the purchasing decision. Already for most situations the battery issue is not important.
It needs to be said that the above statement is, unfortunately, seriously mistaken.
In many areas of the world, temperatures of -20 or below are common. In large parts of Northern America, Alaska and Canada, in much of Europe and particularly in Northern Europe, as well as in mountain areas all over the world such temperatures are commonly confronting photographers. In nearly all of inland Scandinavia - that is Norway, Sweden and Finland - a temperature of -20C is normal during most of the winter and lower temperatures are by no means uncommon. In the inner areas of Norway, Finland and Sweden there may sometimes be -40C and below. This often happens in the northern areas. Photography is performed in all these countries, and is also an increasingly popular activity.
In Russia and Central Asia we know that large areas of the Taiga and Tundra areas are characterized by very low winter temperatures, and we are talking about areas where -20C is considered mild for the winter. The same is true for Mongolia and northern China. While I do not know about photography in these countries, tourism is as we know everywhere and the climate is known to be very cold during winter.
I assure you that many photographers do shoot in such temperatures as mentioned, and that the need for photo equipment to tolerate many degrees below zero (Celsius) is for real. If there is any doubt about that, please perform an internet search after for example "Cold weather photography," "Photographying in the cold," "Arctic photography," "Canadian Arctic photography," and "Winter photography." You will see that many photographers, both professionals and amateurs, take pictures in these areas, and these photographers seem to be growing in number. Electronic viewfinders (as of 2018) drain batteries very fast during use in cold weather. If the temperature is very low, the electronic viewfinder function will soon become so slow that it is no longer functional and finally it stops working. If mirrorless cameras are to compete with optical viewfinders in all types of weather, they have to function equally well in all types of weather. If they cannot do that, they cannot replace SLR cameras with optical viewfinders.
There is no question that a photographic system which is to be a serious one must be able to cope with low temperatures. The equipment simply has to be able to withstand very cold weather and to function flawlessly under such conditions.