Well I'm not a day one player of crpg but I remember when I started to play this mod back in 2011 there was indeed way more teamplay than nowadays.
People were fighting as a group, doing shieldwall, protecting ranged, advancing and retreating together, building stuff (damn I remember those moving siege tower on plains map, now that was funny). EU_1 was a place where tactic could be seen and not only with the impulsion of clan members.
These days we had fewer tools for teamplay than what we have now (commander chat...).
I'm sure it's not nostalgia, those times were better, more players, more teamplay, more fun. (who remember these multiclan battle, that was the shit!)
I don't think the flag system is the one to blame, teamwork was gone way before it's implemetation, for me it's more the player's mentality that changed and maybe fighting ability. Today it's hard to find someone who can't block, 2 or 3 years back, only the best players were able to block properly. Players had to rely on other to survive but everyone improved.
I miss these days too but we won't see such thing in crpg anymore, maybe with melee battleground who knows.