All of that is fine. But put it into context. The iPhone 7 is a direct descendant of the iPod which had its own closed ecosystem. There were all kinds of iPod docks, and if you had an iPod at a party that only had such a dock, you were all set to join the party and plug your iPod into the dock and share your music, and the iPhone benefitted from that legacy. However, if you didn't have a 30-pin dock connector on your device, then you had better have had a 3.5mm male-to-male patch cord to plug into the device's AUX port, because you couldn't rely on the person with the dock to have one -- why would they?
This is exactly the same situation. For a customer who is part of the Apple ecosystem, and to the extent a person wants to participate in a non-Apple device oriented party, they had better be prepared. This has always been the case.
As for any professional DJ being paid money to perform a service, they had better be equipped to address the needs of any customer. The iPhone 7 is no longer a "what-if" -- it's reality. If the DJ isn't using an iPhone 7 themselves, then they had better invest in a $9 dongle to carry around in their kit for just such emergencies at a wedding.
And this has been my experience as an Apple user. Any "professional" situation I have ever been in has been in a position to accommodate my Apple device, be it 30-Pin dock connector, Lightning cable, 3.5mm headphone jack adapter for the original iPhone, Thunderbolt cables, Firewire 400/800 cables, DVI, mini DVI, etc. Considering there might be over 200 million iPhone 7s in the world by the end of 2017, and while 3.5mm Lightning headphone adapters might not be easy to find now in the wild, they almost certainly will be everywhere by then. And this is a typical Apple phenomenon as well. I wouldn't be surprised to find the $9 adapters for sale on some airlines in a year just in case.
Is this the most practical way to approach consumer audio? Maybe not, but the reality is, it won't create an undue hardship for most users who adopt a headphone jackless device, regardless if it's Apple or Android.