Both Patriot (mostly) and THAAD are oriented towards BMD role, its radars, interceptor missiles are optimised for this role. And they are expensive, the latest PAC-3MSE missiles cost almost $4 million while THAAD interceptors cost as much as $10 million (from what i could find). IMO we are not deploying our own system (or doing it very slowly) because of cost reasons, even an indigenous one will be quite expensive given the large area we need to protect.
And I'm sceptical of any S400's BMD claims, especially beyond very short range ballistic missiles, specifications are very sketchy IMO we will only use it in SAM role. Either ways point remains that its still un-proven in either role, over the years I have learned to be sceptical of any over the top claims for Russian maal unless proven in the field.
Patriot is a shitty system.
The last time it was tasked with defending American soldiers 28 ended up day. It has also failed to intercept drones and missiles for the saudis as recently as 2019-20 period.
During the 1991 Gulf War, the public was led to believe the that the Patriot had near-perfect performance, intercepting 45 of 47 Scud missiles. The U.S. Army later revised that estimate down to about 50 percent — and even then, it expressed “higher” confidence in only about one-quarter of the cases. A pesky Congressional Research Service employee noted that if the Army had correctly applied its own assessment methodology consistently, the number would be far lower. (Reportedly that number was one — as in one lousy Scud missile downed.)
Our resident American slave will tell you otherwise but he is at the end of the day a slave. This of course does not prove the S-400 to be the best system but it we are to look at previous engagements like the Israel arab wars Russian air defence units have performed decently if we look at their price tags.
Another point to note is that American equipment comes with strings attached which can be a huge issue. What if for example America and pakistan are buddies again all of a sudden we can see our spare supply choked at the whims of US.
In the end indigenous systems are the way to go. If India wishes to be a credible regional power and later on a Global one we absolutely cannot depend on anybody.