It doesn't really matter how many Mir cards are—they're just a home-grown Russian brand of a card.
The real issue is that all transactions inside Russia using Visa, MC or Mir are processed inside Russia by the same entity that runs Mir brand, and therefore won't stop working.
Which they created in 2015, after we threatened SWIFT removal for the Crimea invasion, along with a SWIFT alternative called SPFS.
It really wouldn't surprise me if the Russian state fully anticipated and expected these financial sanctions from the West, and have positioned themselves to profit from it after some short term pain.
Which, tbh, was pretty much the stated intention before starting the conflict - cut off the new superpowers and their historic satellite states dependence on [insolvent] western financiers.
Very. They (China, Russia, India) are calling it "the multi polar world". Knocking swift, visa, mastercard and the dollar out of every country they can has been a goal for years, and now they are handing it them on a plate. Sigh.
Commenting more in the sense they are supporting non democratic states. The same way companies/consumers(?) are moving out of Russia, they could reduce spending on products from places that fail at environmental, social, governance and democratic levels (it comes with its own set of problems though)
Edit: 25% might be false. Maybe 25% of cards are Mir-only? I'd appreciate if any Russian could chime in to clarify the percentages.