I think you are right - I thought for some reason that Subban was a 2nd year pro. I think it makes no difference - when you bring in Khudobin as the backup, and he's a good backup, his exposure likely protects Subban by virtue of their being 1 more better option out there. Yes, Subban has a lot more upside, but Las Vegas is only picking 30 players, and 1 from each team. Boston is protecting Subban by putting people in front of him.
Think about it for a second - let's say we go with Subban as our backup this year. The result is either he does great, he is meh, or he is horrible. If he's horrible or meh, he's a worse option than Dobby, who has done well here as a backup. If Subban does great, he's a risk to be selected since he is young and cost controllable and proved his skill at the NHL level.
I really don't like any of those scenarios. I'd prefer to wait until next year, and see if he can crack the lineup.