The only non-manufacturing reason I see them waiting until June would be to show off a new, advanced version of iOS for iPad Pro. But would the iPad get the update at launch, or would users have to wait until iOS 11 in September? And if the thing is going to launch in June, couldn't they go ahead and put an A11X in it instead of an A10X? There's also an outside chance that they release the new iPhone at the end of June (maybe just the special edition?) for the 10 year anniversary of the iPhone. Within that context, an iPad Pro launch makes more sense, especially if this new iPhone is branded as an iPhone Pro and has an A11 chip.
It would be nice if Apple released an iPhone Pro a few months before the regular iPhone upgrade that year so that enthusiasts and power users can get the higher end, more expensive model ahead of the typical upgrade cycle. But it needs to be worth the price, having extra features or faster hardware, and not just be a "pay more for early access" type of thing. The downside I see to this is that it takes the winds out of the sail for the "mass market" iPhone unveiling in the autumn.
Given that, if it were up to me—scary, I know—I'd launch new iPhone (regular, Plus) and iPad (Mini/regular) models in August every year and unveil the A_X Pro versions of the iPhone and iPad (10.5/12.9") in November. This means we have to wait a little longer, but I don't mind, and for those on the iPhone Upgrade Program or who just buy a new one every year, their previous gen iPhone Pro would perform favorably (maybe slightly slower or equally) against next gen base models, so there is no real downside. Maybe the iPhone Pro would get the expensive, semi-experimental new features (things in the same "wow" vein of portrait mode—perhaps wireless power, iris scanning, edge to edge display, Touch Bar, full "swimming" waterproofing, @3X mLED display, etc) a year ahead of the mass market version so it gives Apple time to perfect it. Enthusiasts like many of us here would be willing to pay more to get the advanced stuff, better battery life, and more 10 months ahead of everyone else. And for the new components they use, it gives Apple time to ramp up production at their various manufacturing partners for the next year's model as an iPhone Pro would probably only have 10% of the iPhone market share, but with higher margins that help fuel more research and development. Well, perhaps in an ideal world!
It would be nice if Apple released an iPhone Pro a few months before the regular iPhone upgrade that year so that enthusiasts and power users can get the higher end, more expensive model ahead of the typical upgrade cycle. But it needs to be worth the price, having extra features or faster hardware, and not just be a "pay more for early access" type of thing. The downside I see to this is that it takes the winds out of the sail for the "mass market" iPhone unveiling in the autumn.
Given that, if it were up to me—scary, I know—I'd launch new iPhone (regular, Plus) and iPad (Mini/regular) models in August every year and unveil the A_X Pro versions of the iPhone and iPad (10.5/12.9") in November. This means we have to wait a little longer, but I don't mind, and for those on the iPhone Upgrade Program or who just buy a new one every year, their previous gen iPhone Pro would perform favorably (maybe slightly slower or equally) against next gen base models, so there is no real downside. Maybe the iPhone Pro would get the expensive, semi-experimental new features (things in the same "wow" vein of portrait mode—perhaps wireless power, iris scanning, edge to edge display, Touch Bar, full "swimming" waterproofing, @3X mLED display, etc) a year ahead of the mass market version so it gives Apple time to perfect it. Enthusiasts like many of us here would be willing to pay more to get the advanced stuff, better battery life, and more 10 months ahead of everyone else. And for the new components they use, it gives Apple time to ramp up production at their various manufacturing partners for the next year's model as an iPhone Pro would probably only have 10% of the iPhone market share, but with higher margins that help fuel more research and development. Well, perhaps in an ideal world!