I think people have rose-tinted view of old PES games, especially PES6.
I reinstalled and played it again (vanilla, no mods), doing the World Cup mode as a few different teams on the Top Player, and the AI was really not that interesting to play against. The weaker and stronger teams didn't play that much differently, if anything the stronger teams were easier to beat... I didn't have to be very creative on the attack, just zig-zag pass your way up the pitch and then easily round the keeper/cutback pass to score, or just do one of those ridiculously overpowered 90-degree first time shots from anywhere inside 25 yards.
Didn't play any of the ML, but the structure of it was garbage (8 team 2nd division, 16 team top division) and I remember it having loads of stupid transfers - teams would accept any player as part of a trade, get rid of their best players etc. It's not and never was some sort of gold standard for an offline single player game mode in a football game.