I am considering making some extreme limitations upon which iphone models I will work on and which jobs are available. 6/7/8 series iphones will have pretty much all repairs available, but as models get older (such as the 5S, 5C, 6 and 6 Plus), the parts will be available by order only.
I have also decided to not work on "flagship" models (current best models) unless the owner has no other option. If the phone qualifies for warranty or insurance work, I will say to send it to the manufacturer.