I used to restart my games from Gen 1-2 a lot in the past when I still played them. But I have only ever restarted two of my Gen 3 games (which I somewhat regret), and I have never restarted any of my games from Gen 4-6. The reason is simply that if I were to do so, I would lose way too many things. I usually try to complete as much as possible in the games, and since the newer games have more content, it means that more stuff is lost if I were to restart them. This involves things like completing the Pokedex, getting Trainer Card upgrades, winning Symbols/Prints/Monuments at Battle Facilities, winning at minigames, obtaining Pokemon with good IVs/Nature and stuff like that, EV-trained Pokemon and so on. Naturally, if I were to restart a game, I could simply just trade over all Pokemon and items I don't want to lose, but the very core in terms of Pokedex and stuff would still be gone. And no matter how fun it is, I really don't want to do it again after restarting, I'd rather keep it as a proof of accomplishment or something like that. I prefer to buy both games from every pair so I can do everything twice without having to restart, I don't restart either game but also won't use one of them as my "main" (most of the time), instead having about equal focus on both.