5CP best allows the style of play that we've decided upon for competitive play. There is a heavy emphasis on deathmatch and mobility, where one player can turn the tide of a fight yet cannot do so without the support of their team. 5CP simply gives 6s teams more freedom in terms of mobility and what teamfights to take etc. By all accounts, we can run other gamemodes in 6s (Payload and Attack/Defend tends to be the ones brought up most), but it becomes a question of whether or not we should. Does having more gamemodes create the sort of competitive game that we want? Are what makes comp tf2 good promoted in these modes?
That isn't to say that 5CP is flawless. Unlimited rounds with a round timer reset per cap means that stalemating is an option that doesn't have any significant downsides whereas stopwatch necessitates constant action. In order to create the 6s meta we wanted, we've had to cut out certain classes from being generally usable to prevent 5CP from being absolute torture to play which can be seen in HL or even Casual matches on these maps. Fixed match limit creates potentially 4-5 minutes of garbage time which is unfun for players and spectators alike. But we put up with these flaws because we've determined that the alternatives are worse.
And again, 6s has its issues. However the community decided that 6s promotes the best of what TF2 has to offer and the meta we run is just one consequence of that. Sideshow made such a point in that video, in that the meta isn't what is 'best' but the one that is the most fun or conducive to our preferred playstyle. The one flaw I think should be addressed is encouraging a bit more flexibility in classes/weapons outside of just last holds, but the challenge is how to do that without breaking what we already have. Without external incentives, there's little reason to shake things up for the hell of it.