I buy games all the time that came out on other platforms first. I may be disappointed from time to time for having to wait, but I would never refuse to buy a game I am interested in because of it.
I don't think that it's an outright refusal based on it coming out somewhere first as much as it is the fact that a lot of your first-day purchaser crowd will have probably played it already if it was on PS4 and PC for a year before it hit X1. Even if they didn't, hype sells, and there is
usually very little in the way of hype online for these late ports because the hardcore crowd has played it and moved on. People are focused on the new hotness. The thing is, even if released at the same time, most of that same hardcore, first-day crowd will be playing on PS4 or PC anyway due to better graphics and the "my library's here" effect, but the hype and activity they generate online will help the more casual gamers jump onboard with whatever system they have at home. That activity would help the X1 version to sell better, regardless, but only if it's there for people to buy. If it's not, then those users just get sad.
Furthermore, they will be releasing on X1 at a "new" price when the game will already be discounted (perhaps heavily) on the competing platforms. Even if they don't own the other platforms, people are aware of the pricing, and it's distasteful to pay $14.99 for a year old game when the other platforms have already discounted it to $10 or had a sale for $5. I've seen threads in this very forum where people have made comments to this effect: "A year old game for full price? LOL no." or "It's only $10 on <competing platform>. WTF?!". However, I don't think including pointless extras is going to help this problem in any conceivable way. Maybe it would be better to simply suggest parity pricing at the time of release so that doesn't happen. The port is going to suffer for being late no matter what but there's just nothing MS can do about it.