But we get spin-offs and Pokemon related games like every year.
Which are really easily ignored (unless it's Pokemon Snap...we need more of that) even by those who play the game habitually.
Looking at the actual franchise on the handhelds, oversaturation actually happened more toward the beginning with Yellow and GS coming out within a MONTH of each other and only a year out from R/B. Crystal came out a year and a half later, and R/S a year and a half after Crystal, which means that the legitimate followups to GS came out over 3 years later. FR/LG came out a year after that, though being a remake and part of the same Gen, you could've ignored it if you wished. Emerald came out 2 years after R/S. D/P came out 2 years after Emerald and FOUR years after R/S. 2 years later is Platinum, a year after that HG/SS (same deal as FR/LG) and then a year after is B/W.
Breaking all of that down, first thing's first, the quickest turnaround from First Two Games to Trio Completion came from R/B into Yellow which happened within a year. After that it's usually a Year and a Half to 2 Year Wait. Now, if you ignore the Trio Completers and Remakes, the usual downtime between new Gen Poke games is 4 years, again the exception being RB into GS which happened within ONE year. If you ignore the first two games and ONLY get the Trio Completers, the gap is again, 4 years outside of Yellow to Crystal which took a year and a half.
Long story short, unless you REALLY want to buy EVERYTHING even when it's not necessary, you'll only have to buy a Poke game every 4 years at best, every 2 at worst which at is core is a lot less douchey than most Sports and Activision games.