Mini-14's are too expensive to produce for the Military. Recycling beercans into small arms is what they look for. The M16 and variants are "good enough" for most military applications. They are by no means some kind of high tech superior weapon.
There are a number of things that make the Mini-14, or full auto version, not suitable for a miltary rifle. The magazine design is one thing. The inability to replace barrels easily is another reason. Ruger never really attempted to keep up by inventing new Mini-14 tactical platforms.
Three things that could really improve the Mini-14:
1. modify the magazine and mag well to make it quicker and easier to swap out magazines.....i.e. get rid of that hole-catch. Better yet - just make it so that it uses AR magazines.
2. Make it relatively easy for non-ruger armorers to swap out barrels - I don't know how this would be accomplished, but it can't be impossible. I can think of several designs off the top of my head using a ratcheting system, and a retractible gas port/tube that pops into place when the barrel is screwed in completely. Accuracy and ability to handle sustained FA fire is really a factor of what barrel you put on the gun.
3. Ruger would need to license the production of parts to other companies and permit them to sell parts for their rifles on the open market.
If Ruger could do these two things, It would have a real "winner". The SCAR CQB stock proves that you can drop the action into a tactical stock that provides more tactical options than does the M16 platform.