Did I say "since S-400 is expensive ... it will be left defenseless?"
I said, "S-400 is expensive, so India will have a finite number of them." The meaning of what I actually said is that because the S-400 isn't cheap, India won't field a large number of them. In reality, the S-400 will be part of India's wider air defence network, which will include other medium and long-range SAM systems.
Now, to be clear, I didn't say it will be left defenseless. I said, because there are comparatively few of them in India (or will be), Pakistan's best bet will be to destroy them. Nothing was said about it being easy or the S-400 being defenseless, I just raised an obvious fact - to neutralize something, you need to destroy it.
In fact, I even said "it won't be easy." ... but the tried and true ways to deal with the S-400 problem is to locate them, and then try to use stand-off munitions such as cruise missiles to destroy them as quickly and abruptly as possible. India has every chance perhaps to move them in time, so the task of destroying the S-400 isn't easy by any stretch, it'll be very difficult, but it isn't impossible, though it will require a lot of assets (e.g. IMINT, SIGINT, etc) to have a shot.
To be honest, I am not sure why people jump to the high horse so quickly (?). If you have a problem with my point, you had every opportunity to just ask me about how or why I got to my conclusion. Why jump to a snarky attack? What's your problem?