Context is important if you're talking about the game industry in the 80s/90s, which is very different from today.
Also you need to understand in Japan having a monopoly is not illegal or even considered bad. In fact, it's considered a compliment of your business acumen, lol (this is from the Console Wars book). So Yamauchi would have viewed everything that he was doing back then as perfectly fine.
If you called Nintendo "monopolistic" at that time, it was a compliment.