Hint; All gear gets replaced, besides precious' ribbon.
I understand the jump in ilvls, I really do. VP upgraded gear should be considered the "top" gear and the increase should be in relation to that, because you want to replace gear from new content. It makes sense. The problem I have is that VP upgrades exist and are really the reason for this drastic jump from non-upgraded t14 gear to non-upgraded t15 gear. Now I may be going to crazy town, but please bear with me.
Justice upgrade and Valor upgrades serve a purpose: to increase your ilvl. As Ashunera stated in his blog, this game is laden with ilvl requirements, most of them being a restriction to prevent under-skilled players from waltzing into content they're not equipped to handle (and to slow character progression to keep you from reaching "the end" too fast). When it comes to upgrading dungeon gear and LFR gear, I agree with that. It gives non-raiders a way to progress after they've reached their limit to obtaining gear for that week in hopes of achieving that next ilvl hurdle. What I don't like is having to upgrade normal and heroic gear with VP just because it's available. Having to choose between upgrading a piece that'll increase your dps or waiting to see if you'll replace that item before upgrading it is silly. It's not rewarding when your RNG coin decides to give you that staff you wanted right after you spent a week and a half's worth of dungeon grinding to upgrade the old one. When you're raiding in normals or heroics, you generally don't have the ilvl hurdles to deal with anymore. The gear that drops is significantly over any of the current restrictions, so now you're just upgrading for the sake of upgrading.
I really think the way JP and VP upgrades work should be reconsidered. It's a great way to get past those ilvl restrictions when you're just starting out, but a chore once you get past that. It's causing an unnecessary stat bloat for normal and heroic raiders, making certain current content absolutely trivial once you've upgraded the appropriate pieces (any heroic raiders that have gone back to heroic MSV can attest to this). Really, I think JP and VP upgrades should only apply to heroic dungeon gear and LFR gear. Eight ilvls on an LFR piece is still below normal, so there's still an upgrade available even after you've upgraded your piece and you've cleared your ilvl hurdle.
With normal and heroic gear being excluded from VP upgrades, you could stop a lot of the bloat happening down the road. Sure, there isn't much to do about T14's already upgraded gear, but you could nip it in the bud now and save the hassle later. Maybe even drop the ilvls down a good deal, making LFR drop 496 gear, Normal drop 517, and Heroic drop 530. That way non-upgraded heroic T14 can still be replaced, upgraded heroic T14 has the potential to be replaced (due to more-desirable stats), and gear won't be as incredibly inflated with stats.
It's a dangerous road we're heading down. Some specs have hard caps that will be easily attained by T15 and most definitely by T16, leaving their spec's value at the hands of loot itemization. It genuinely worries me.