The problem is license costs. IBM makes a great system but almost all their customers have been working hard to replace it, not because of issues with the quality but the high license costs.
There's really only one acceptable price for an OS, and it's the same as the only acceptable price for a browser or DB - $0.00. The precedent has been pretty solidly set (Linux, FreeBSD, MySQL, Postgres, Chrome, Firefox...) at this point, hence all the pivots to charging for service and support and suchlike.
IBM AS/400 lineup was fully designed to optimize customer segmentation and threfore capture better the customer value. For example, there were cheaper models where the hardware was exactly the same as more expensive models except for additional hardware loops to make things run slower. That’s why they had so many models, they wanted to have a price point available for every company size.