I admire your cost fixation Smile The Su-35 is currently in production, whereas the MiG-31 wasn't manufactured after 1994 . How would you envision upgrading and continuing to use airframes that are already on the verge of being worn out?
The MiG-31 is no light agile fighter pulling 9g... it would take off and accelerate to speed and altitude and fly rather fast to a point in space where it would launch its long range missiles. It might then descend and chase down any missiles its target managed to launch before being shot down or it would fly back to base to rearm and refuel to get airborne again to hunt for other targets and any small low flying targets that might be trying to sneak through.
There are no sustained high g dogfights, or heavy stress on the airframe, which is rather heavily built in the first place.
A motor car... without collisions, doesn't require the chassis to be replaced after x hundred thousand kms of road travel.
Of course with the SR-71 they found the titanium was actually in better condition after years of flights because the massive heating and then cooling of the airframe with each flight actually further hardened the metal and actually made it stronger with use.
The point is that there are MiG-31s in storage that have not been used and those aircraft can be taken out of storage and be given upgrades and new engines and will be good for 20 years if they needed to.
During the original discussion, as far as I recall, the Russian Ministry of Defense stated that there would be no restarting of MiG-31 production but rather the intention to design and build a new interceptor. In the meantime, they planed to upgrade (MB and K) and continue using the MiG-31 until they reach the end of their operational lifespan.
And if you watch the video above it mentions a few dates that have been mentioned before like 2028 as being a time when some of the more heavily used MiG-31s might need to be retired and a replacement aircraft be put into service to replace them.
They had MiG-31s in storage and have recently put the engine back into production to take some of those aircraft out of storage. Planes that have been in storage and get upgrades and new engines wont be retired in 2028, but some aircraft have been in use for quite some time and those aircraft could be retired and replaced.
There are plenty of options for upgrades including replacing the radar with a very powerful AESA radar array that would be rather enormous and very very powerful.
They don't need to make more MiG-31s when they have some in storage and are making the engine... what they are going to do it make the replacement and once that enters serial production they will likely slowly withdraw the MiG-31BMs from service... the ones with lots of airframe life left they might convert to MiG-31K duties or just put them in storage and use them in areas where MiG-41s are not needed yet.
Also because majority of russian legacy planes sat instead quite idle for more than a decade due to Eltsin's era economic downfall.
There was quite a period of time there when not many aircraft were actually flying and many were put into storage.
AFAIK they have however a lot of model DZ available , so they plan to get an interceptor version of them also, beside the K one.
Another factor is that they will be testing new systems and equipment... some of which it might be possible to fit into the MiG-31 for testing and actual operational use... more sophisticated EO systems... perhaps its own wing mounted long radar wave radar system to look for stealthy objects and platforms... and improved radar types.
A good way to get real world experience with a system before taking the risk of launching an aircraft with it as standard.