For starters Turkey does not have any money to buy F35s. I don't even know if they have the money to spend on useless (to them) S400 or patriot systems. The country is in recession, inflation is north of 25% and people there are struggling to survive.
The Kurds that Turkey is fighting don't have any aircraft. So would a Turkish Patriot or S400 system take aim at? Greece, Armenia , Syria, Iran or Iraq maybe. Take your pick.
In due course, F35 radar signatures will be calibrated into the S400 system but the US doesn't want to accelerate this process. The US is also concerned that Russia would find a way to share or otherwise obtain F35 technology through Turkey.
Adding to the craziness, Turkey has a thing going with Al Qaeda (HTS) in Syria's Idlib Province. It was always Erdogan's goal to establish an Al Qaeda ministate in Syria and that objective has been achieved.
[For the benefit of ignorant Trumpists out there ISIS was an Al Qaeda off-shoot and so is HTS. They are all one and the same thing. These extremists continue to control significant territory with Turkish acquiescence and support.]
