I don't really know if having this many young D is ideal for a team that's trying to "win as soon as this year", maybe McDavid is so good he can overcome that, but that's an awful lot of inexperience.
The trade value a Zegras would have would be enormous if he played with a McDavid or Draisaitl, that has to be factored in too, Arizona would probably have to add to Chychrun for example to get Zegras if he was putting up 80+ points here, which I don't think would be unreaosonable. He's a 60+ point player in Anaheim, now factor in McDavid or Draisaitl being on his line.
I think you always take the BPA, you shouldn't ever draft based on positional need. Now hopefully for the Oilers Broberg is a decent player himself, I'm not really sure if I see the upside of a no.1 D there, but at least he's useful right now whereas earlier in the year that wasn't the case.