I've already explained how this is different from existing Russian-origin inventory in NATO. For example, Poland wouldn't be operating MiG-29s if they didn't have a pile of them already, and they have been updated to work with NATO equipment. Obviously, simple Russian-origin equipment like guns or trucks are less subject to compromise than a highly networked air defense system and are therefore less of a concern. The Erdogan-Russia propaganda line you may be searching for is "Greece has S-300's!" But these are from the mid-90's and obtained in an awkward transfer from Cyprus, a very different geopolitical situation.
What else am I supposed to back up? Even if I did know the inner workings of NATO and Turkish defense networks it's not like I could write about it online. What I do know is that buying military equipment almost always entails close work between contractors and military personnel to set up. I know its a risk to let adversaries near your equipment, the contractors in this case. This is very common sense operational security. You don't invite foxes into the hen house.
I'm not making these concerns up, either, here is that G
en. Petr Pavel, chair of NATO Military Committee, had to say in March 2018. He's Czech, by the way:
Like I said, Turkey isn't out to give away intelligence to Russia. Even if they don't want to give away any sensitive information, they still are giving Russia a potential intelligence platform on their soil.
The potential sanctions for India stem from a couple itty-bitty oopsy-whoospies the Russians committed like invading Ukraine, assisting Assad in Syria, and interfering in the US Election. T
he US is deliberately not applying those sanctions to Turkey. Defense sales is probably also a factor in India but again, the US is
torching opportunities for defense sales in Turkey. The actions against Turkey do not make any sense from an economic angle.
Turkey has a much better guarantee of safety from Israel and Saudi Arabia by being a US ally, especially one codified through NATO. This concern over air defense is nonsensical, the threat's just not there if you have NATO backing. It simply doesn't make sense to buy the S-400 unless you are Erdogan and trying to suck up to Russia.