Even if they matched price with PS4 by Destiny's launch (which is unlikely, unless they drop Kinect), they still have to contend with the stark value proposition that most of the market agrees is an issue. If it matched PS4's price, it's still got the worst version of multiplatform games and still is going to have the worst version of Destiny, if that was a motivation. It's still going to have apps all locked behind a paywall and for hardcore gamers, on top of the power issue, it's got the extremely damaging ID@Xbox policy still in place.
Xbox fans have left the Xbox infrastructure in droves already. If people haven't seen that already, it's only going to become more and more dramatically obvious as this gen goes on. And anecdotally, I was a huge 360 fan, spent 90% of my last-gen time on there online and off, purchased almost all my network/indie games on there, and almost my entire group that I played with religiously on 360 has left (except for my friend Eugene, he refuses to get any Sony system because he had a bad experience with Sony customer support back during the PS2 gen. Even though he had seven Xbox 360's break. Whatevs). This is a story that is repeating over and over again.
Destiny is going to sell the most by a substantial amount on PS4. It is going to be marketing to look -exclusively- PS4 in the same way as Titanfall was marketed to look -exclusively- XBO. They're got a huge deal on the advertising, they've got exclusive content, their version of Destiny is going to look much better. Any Bungie fan is going to want to play Bungie's vision at its best possible quality, and the most convenient thing about that is they don't even have to pay more for the convenience. They have to pay less. No amount of Xbox fanboy Halo loyalty is going to overcome the onslaught Sony and Activision have planned to shroud the Destiny legacy.
Wow, this one looks fucking hot. Totally want that.