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It's been a steady decline. In Gen I you were rewarded by Oak's Aide's for PokeDex completion, and either there weren't any in Gen II/III or they were so insignificant as to not matter to me.
In Gen III catching them "all" wasn't feasible to most players thanks to so many being locked behind the Gamecube games
Gen IV switched the emphasis to "seeing" them all, which seems to be Ash's aim
B2W2, XY and ORAS all give the shiny charm for catching them all, which is nice
Gen VII literally doesn't have the national dex so you wouldn't know if you did anyway
