The single-player doesn't have to be over just because the credits roll. What comes after to me is really what drives home how great the game is tbh, because it's something that the game doesn't expect you to do and places no restrictions on you but when you start the postgame and it's great, it feels extremely gratifying.
Postgame can and usually does mean single-player experience when it comes to Pokémon. Exploring Kanto in HGSS (GSC had technical limitations) did more to help the world of Pokémon than anything that's come since. There's a region with a history that's moved on from Red/Blue's story three years ago, and you can feel it everywhere you go. It's not perfectly balanced from a gameplay standpoint, but that's not the main thing I look for in Pokémon games. It's about the adventure.
Similarly, Sinnoh's sizeable postgame island makes the entire world of Pokémon feel so much bigger, and Sinnoh is already the most massive region alongside Hoenn.
Even a game like Black 2/White 2 felt huge to me because there's so much to that game that's optional because of how much they added to Unova from Black/White to make it a fully-fledged sequel. Obviously, that game depends on you playing Black/White first to really enjoy it.
When you don't have a postgame, or really sizeable optional content outside of the main story, the world of Pokémon just feels like set dressing for stories that on their own really aren't great.
The Lucario movie is really good and has an awesome setting. I like Heroes for the same reason and felt like Latios and Latias perfectly fit that location instead of just invading it like so many other Legendary Pokémon where the Pokémon themselves are always too much of a focus.
I saw some pretty good fanart from Lownine for it. He's a really good artist when it comes to depicting scale in the Pokémon world and adds a touch of believability to his artwork.
When I was a kid and I got all the badges I was dedicated to getting all the Pokémon and I did. I don't have an urge to get over 700 Pokémon. I find that ridiculous number.