As a youth a few decades ago I had an opportunity to play with one of these systems at home (and I actually still have it boxed up somewhere). I managed to acquire a 9401 Model 150 that fell off the back of a truck, it came with a 5250 terminal and a heavy box of documentation. Even as a kid who had started playing with linux in high school I really struggled to get it to IPL - the system was seriously weird from anything I had known before. Strangely enough, within two years of that I ended up working for IBM and went on to be trained on servicing AS/400 and iSeries systems. What was fascinating as an outsider coming in to IBM was just how hard they clutched their pearls on the AS/400 system. I also serviced RS/6000 (pSeries) and RS/6000 SP clusters so I saw two different sides of the house operate. I was a typical pimply faced youth eager to learn, and the pSeries guys, both in the field and in IBM training schools were happy to feed me all the info I could digest. When I needed software for testing stuff on my own personal AIX box, stuff would get passed my way no questions asked. On the iSeries side, they would quite literally tell me "no" and shut the door in my face if I was asking too many questions. Despite the fact that OS400 can't be ran on any other hardware than IBM's, IBM guards the SLIC or OS "golden tapes" very jealously. The class for learning how to service iSeries was four weeks long back then, and our brains got crammed full of tons of information. But if I asked questions outside the scope of servicing the hardware I'd still get the door slammed in my face ("that's a different class kid, your manager will have to approve that").
I only worked there for about five years and I am grateful for the experience. I'm also glad I left when I did to avoid becoming "institutionalized" in my thinking as I had witnessed others had become. I have to assume that the pearl clutching came from IBM's experience with Amdahl and its prized mainframe (zSeries). They had become so afraid of competition that they were even afraid us lowly CE's would start freelancing and selling consulting services on the side on OS/400 work. As a result of that, you have a walled garden which is very hard for fresh talent to penetrate and IBM likes it fine that way.