Oh no I definitely get "the idea", but I think you're missing the point here. Speedrunning is a highly refined and charted (as well as risky) way of playing through the game, but if you add enough minor variations to it and mainly just preserve the one Pokemon aspect, you end up with a very casual method, which is one that is very "regular" and also very efficient. So many casual players I know just played with one overleveled Pokemon the first time, with the logic "well if what I'm using is working, why add other team members?"Because the idea of the in-game tier list is to rank Pokémon according to their merits in a "regular" playthrough, and use that as a yardstick for their relative usability in other forms of play. Single-'mon speedrunning is a very specific and specialized way of playing, with its own unique requirements and conditions, and is pretty far removed from a casual playthrough, or most self-imposed challenges.
Also, speedrunning is not the mode most people on here play. There is an old speedrunning thread around somewhere, but it fell to the depths of the forums because of inactivity. I hardly ever see people posting about plans to speedrun a game, mostly we talk about playing on a way more casual level, and the in-game tier lists are most useful when written accordingly. I think separate speedrun tier threads could thrive given enough interest, but currently it appears to be a too "hardcore" mode for most people in here.
It's past midnight, so I'll stop for now, but hopefully you get the idea.
I am not endorsing a speedrunning-style. But what I will argue is that (at least for Gens I-IV where the experience formula does not take into account level differential):
- a 1 Pokemon strategy is efficient and effective (as evidenced by speedrunners)
- a 1 Pokemon playthrough is just as "casual" as any others. In fact more, because you're not constantly switching around, just hiding behind one Pokemon and dominating.
- a 1 Pokemon playthrough is just as "regular" as any others. In fact, I'd wager a lot of money that more players instinctively choose something resembling a 1 Pokemon playthrough versus any other specific amount (2-6 Pokemon)
- what is in fact extremely inefficient, is using playthroughs as a "yardstick" for usability for other forms of play. They are two metagames with very limited overlap. Even individual Pokemon, as a general rule with a few exceptions, have a very limited access to their full movepool that they would use in competitive play. Think of egg moves, tutor moves, tradeback moves, difficult to obtain/non-replenishable TM moves, etc.