The Reason Why Not Every Strawhat Will Get Haki

First, I don't think Oda wants to homogenize the power base of the main characters and I feel that he still wants every character to have their own distinct ability.

Secondly, and most importantly, the SH's are not going to be staying in the NW ruling over territories like the other Emperors. It's pretty clear that the 'Three Great Powers' will crumble by the end and be replaced by something new. If the SH's follow Roger's path, like they have been, they might even disband and settle down in different places.

The New World is teeming with haki users for the same reason that the early GL had more DF users than EB. It's because the strongest of the strong are fighting for control and majority of the characters are NW inhabitants who have studied haki.

Yes, haki is useful in battle. Whether it's against DF users or whatever it is the HK can do. But Oda has shown that non haki users can best haki users; Luffy and the SH's took out Eneru and his Priests pre timeskip without haki and Luffy alone took out two of the Boa sisters without using haki.

However, when it comes to final stretch of their journey; the trip to Laugh Tale, what's more important will be the crews non haki related skills like Nami's navigation, Robin's knowledge of the Poneglyphs (Rayleigh even said she might reach a different conclusion from the Roger Pirates) , Franky's ability as a ship wright and Chopper making sure nobody falls ill like Buggy did during Roger's trip.

Also, a big part of WCI was how important the skills of the non combatant were. Judge was basically a parody of your average power scaler who didn't believe that Sanji's non combat related skills were worth anything. Only, it was Sanji's compassion that ultimately saved Judge and his family and it was his culinary skills that stopped Big Mom's rampage.

So yes, haki is important but it's not the be all end all. It will be necessary against certain opponents. But I think for some of the more critical portion of the SH's journey, the non haki non combat related skills will prove to be the most vital.