Go play CTF for any other game and then tell me TF2 CTF isn't slow. Part of the reason TFC worked with instant respawn was because of exploits like bunny hopping and conc jumping (not to mention the shere amount of spam, mirv's = dead sentry). These games are so fast you barely see people. Also, look at Quake, or Red Faction. Most weapons kill in 1 hit and you 'walk' at sprint speeds. Even with 20-28 second respawn times, the slow class speeds do not allow players to get out of the base before they have to kill the same people they killed on the way in, on the way out.
You're lucky if you manage to 2 shot someone in TF2, then there are medics and dispensers. Obviously these were present in TFC, but their mechanics are completely different.
Besides, 50% of flag captures seem to be when one person sneaks past the defence. I usaully tell people not to follow me in until i reach the intel room because "they make too much of a racket".
I'm not saying teamwork is irrelavent. Obviously it helps, and can improve your chances of a successful assault. But it's as likely to cap the flag either way. In the case of TF2, a weak defence is a strong offence as your enemies over stretch their forces. Working together focuses combat too much that no one moves anywhere for half an hour. ctf_2fort is a real gameplay enigma, where it pays as much to not work as a team as it does otherwise.
Capturing the flag is largely a matter of luck, fortune that your uber pops before theirs, fortune that their engi was not watching his gear, fortune that their pyro was assaulting your base while your spy assaulted theirs, fortune that their respawned soldier took a left onto battlements as you traverse the ramp room to the intel, fortune that you got the random crit instead of the guy shooting you.