If Ruger would begin selling Minis that use generic AR magazines they might sell a lot more. Having a comparatively expensive and proprietary magazine in today's market limits the sales potential. Once the AR-10 type rifle makers standardized on the P-mag it made selling them easier.
Tapco gen 2 magazines work at least as well as Ruger mags. $15
https://gunmagwarehouse.com/tapco-ruger-mini-14-gen-ii-223-5-56-30-round-magazine.html
I think the mini looks best with a 20 rounder