I do believe all of the things mentioned are true. With lower tiers, I think it would require more thinking to come up with strategies to make a Pokémon better at what it mains in. With OU tier Pokémon, a lot of what they are made up of (stats, movepool, etc) are easy given and require less thinking really. It's like a comparison between a rocking chair and a recliner as I mentioned with Yoshikkko. You can fix up a rocking chair if you want it to meet your satisfaction, whereas the recliner is already made easy for you. If you're into strategic thinking and want to actually have something to be pretty proud of, I think lower tiers allow more of that.
Though I can see it the other way around, why work harder than you have to? OU is "advanced" and therefore used more, so why not get into it? More people play around it, so it makes more sense to join in too. This is like a bandwagon except it can't be stopped from happening. It's simply "better" in terms of just going all out there with battling. The Pokémon in this tier are simply better. Lower tier Pokémon can only be made as good as they can within the tiers they are set in, which are all lower than OU. But it's all in the trainer that decides how well they can do in any tier, even if you mix it up a bit and add in some lower Pokémon into OU. There's so many ways to view this, that I can't argue with any opinion on this.
Nevertheless, LOWER TIERS FTW > I find it way more enjoyable because there's more to be proud of when you work up something from scratch rather than go the lazy way out.