I think people mostly look back at it semi-fondly almost in spite of its shortcomings, because, to me at the very least, it was a gen where you had agency. SS wasn't that, with its Clefable cores and Scald spam, amidst other things that trended on the bulkier, slower side, where it wasn't a battle as it was a war of attrition. Back then, in SM and inarguably now, you were and are able to make different teams, depending on what you wanted to do. Rain? Slap a pelliper on your team, build around it, and have fun. Sun too, bolstered by Solar Power Charizard and Protosynthesis.
Its looked on favorably because its a more brighter past than the recent slog, and why I liked it, as i do this meta. I hope things develop well, and not towards a slow, stalling meta riddled with fat unbreakable beasts.