The problem is they don't learn. They whine on the forums about how hard things are until SE nerfs the content so they don't have to actually learn anything. The duty finder is largely pointless for a lot of content simply because so many people refuse to even try to improve and thus groups are incapable of clearing content. A lot of groups will wipe once and disband, and it's usually the 'bad' players that leave first when they realise this group can't give them a free carry.
Some form of solo content that forces players to engage with their job mechanics and try to better themselves would work well as a gating mechanism for access to endgame content. They wouldn't need it for access to roulettes or other content where their impotence won't wipe the group but a skill-based check of sorts to unlock raid content like Alexander in the DF would be a good thing. They would have no reason to rage if they can't clear the gate because they wouldn't be able to handle the content that it gates anyway. Something akin to FFXI's Maat fights would be ideal for this.