So having an interceptor is good, definitely the sites in Poland and Romania don’t really provide enough coverage. But, the interceptor is only one part of the defense. Before you can use it, you need an early-warning sensor which can spot the flare of a missile launch on the ground (or at sea) - most effectively done with a network of observation satellites. Then that sensor needs to hand off its data to a tracking radar system - preferably one that can track the missile from its boost phase through the atmosphere all the way up into its sub-orbital path. You will probably need several different radar systems at different locations with different angles and ranges to do this effectively (all actively sharing data with each other).

Modern ICBMs are nasty things with multiple warheads and also multiple decoy warheads, and they’re constantly dropping off empty fuel tanks and cowlings and other bits of hardware to shed weight during flight, so you need a highly sensitive radar to discriminate among the various debris and identify the real warhead(s). Once you’ve got that, you can track it for a bit to determine its trajectory and then you can feed that data to an interceptor system to hit it.

Also, explosions aren’t worth much in space so your typical interceptor uses a “kinetic warhead” which is basically just a solid chunk of metal (it’s a guided, rocket-powered bullet). You have to hit the target directly. If you miss by half a meter, you missed.

All of this identification, tracking, discrimination, targetting and intercepting needs to happen within the very few minutes of the ICBM’s flight path, preferably before the warheads separate and spread out. The point being, it’s a very difficult thing to actually accomplish and requires a lot of precision, and many different technologies working together in real-time, which is why I say that the MDA’s current system couldn’t be replaced in less than 20 years.