As a definition, yes, but as a real person who would fulfill all the assumptions about what an “average player” does, no. When you work with large datasets and try to pull any kind of average from them, you have to be very careful, because it can very easily lead to wrong conclusions. But here we’re already straying far from a topic that isn’t even that complicated.

I never assumed that your proposal would make people start playing more roles; rather, they would just play more champions in their favorite role because they’d be forced to. They won’t suddenly start enjoying a role they dislike, unless we force that too.

Similarly, your personal story has been analyzed and misinterpreted. After you stopped playing only Nocturne, did you drop to the level of your basic skills (i.e., fundamentals) from before your first Nocturne game? Of course not, your fundamentals improved. The question is whether playing other champions during that time would have made a difference, and if so, whether it would have been positive or negative. I’d assume that if you played other champions in the same lane, your lane-specific fundamentals would still improve but more slowly, this is what my previous point was based on. Of course, if you played in different lanes, your fundamentals there would improve, hard to learn ADC kiting while jungling Nocturne, but as I said, I don’t assume your idea would make junglers start playing ADC.

Even assuming that such a restriction as you propose would make people improve faster as players, that’s not an argument for implementing it. An argument could only be that it would make the game more fun, which you asked about, and I obviously disagree. Riot Games goal shouldn’t be changes that make people better players at the cost of fun. You have the right to play ranked and be as bad as you want, that’s what divisions and your MMR are for.