The argument for Arena:Respawn makes less sense than the argument for 6s. At least most of the 6s maps are default maps and at least 5cp and koth are official game modes which are played on pubs semi-regularly. Hell, even some pubs I've been on have class limits, so it isn't like class limits are some foreign concept to your average pubber. Most pubbers understand that demos are extremely powerful, and I've seen plenty of pubbers complaining about certain weapons that are banned in 6s.
Arena Respawn not only includes class limits like 6s, but also adds a bunch of stupidly convoluted rules and game mechanics that are more frustrating than fun to play with. Along with that, it also utilizes a game mode which I bet 90% of the TF2 community hasn't even touched. A pubber would probably have less trouble if he was thrown into a random 6s lobby than if he was thrown into an Arena:Respawn game. Not only is full of a bunch of messy piles of random rules that seem like they were thought up by throwing darts at a dartboard full of options, but it is often even more frustrating to play than 6s. Just because the games are fast paced doesn't mean that they are fun.
Quite frankly I'm extremely disappointed that the most popular competitive TF2 Youtube channel is supporting such a bizarre game mode which feels even more pubby than most pubs, especially since the great majority of the community is against this game mode change.