It is implied throughout your entire journey that THE (singular) champion is THE (singular) strongest trainer in the region, and that defeating them makes YOU the new champion. In fact, when you do beat the champion, it tells you in no uncertain terms that YOU are the new champion. However, in every single postgame, you get a ticket to go somewhere that only "someone who defeated the champion" can go. And sure enough, it's an area FULL of champion ranked trainers. The frontier brains in gens 3 and 4, the entire Sakhalin part of Sinnoh, Johto just gives you access to the Kanto region, Gen 5 gives you half the region etc. The only game where this makes any sense is S/M, but that's only because they actually make you play as the champion and defend your title. It makes no sense.