I generally wish for mainline series to be pushed on (at least!) 4 years gap, since IMO it will become very hard for GF to consistently push out a new gen every three years and keep game sales of Sword and Shield in the long run, something like a fall in sales like in gen V can happen, and i really do think that would benefit the games (the space between mainline entries, not the fall in sales). In a 4-5 years time the technological leap is bigger and more obvious, and in the meantime they could make more side series games on current generation engine. Side games is where IMO the franchise is suffering since the departure from the 3ds on the switch, and putting out more of them in the time between the generations would drive up the hype for the next gen, but that is besides the point. Consider how will the gen V remake play out, which is GFs least favorite gen since its the worst sold gen. Too me, BDSP felt like a "Yeah, it's mandatory remake time" while it could be HGSS level Remake, but they are not doing it, because the new game needs be put out on the announced date, so the development of a newer one can begin. GF still did not fully adapt to the 3D era, and i don't think that in terms of graphics quality, but rather in that it takes more time to develop a modern 3d game, as it was mentioned in the previous posts. Consider that bewteen Rockstars GTA V and VI there will be at least a decade of time between releases. They could be making more Legends and similar games in older regions, without the need to pump out new gen with new mons every 3 years. There will soon be a thousand mons and 9 regions, and the latter feel more disposable as the time progresses.
But the thing is, GF makes more money off toys and merch than games, so they will just keep on pumping them out every 3 years, I guess.
Edit: I also think the releases are correlated to the lifespan of a nintendo console, one is a early console game (Gens 4, 6, 8) which makes fans switch (no pun inteneded) consoles, and the the other one is the definitive gen on the console game, when the game has bigger install base and devs are more familiar with its capabilities (gens 5,7 and 9) when Nintendo is already in the final phases of creating a new console.