The argument that HL is a stepping stone to 6s is ludicrous.
The amount of prerequisite knowledge of how to play a map that's required to compete in HL in comparison to 6s makes it much harder for new players.
The way that each class has a different value in highlander makes it much harder for newer players to understand whether they have an advantage in a fight or not.
I agree with enigma's point, because I feel that there will be people who tried to get into "competitive" by playing highlander and will have lost and not understood why, which can be frustrating. I have first hand experience of this with one of the first teams I joined doing a lobby stack and getting destroyed by Tseini, the team disbanded after.
In 6v6 new players will get killed purely from being alone or losing fights. It's very easy for new players to understand that they had to take a 1v3 or that they lost a 1v1. It's not very easy for a new player to understand why the enemy sniper is able to get into positions to peak your medic, or why you should push from certain places depending on what picks you have.
That being said I prefer highlander for the strategy and teamplay, and if you want a properly accessible gamemode for new players you should probably do ultiduo tournaments.