What few people remember is Reagan offered to share the SDI technology with the Russians (Gorbachev) to prove they could trust him and the US.
"Trust but verify" did work both ways and the Russians dealt with Reagan with respect and trust, Reagan returned the same with inspections for both sides, by both sides.
Not a shot was fired, not a shot was threatened, and the Soviet Union (and the Warsaw Pact) went away because it wasn't necessary.
Now look what Clintoon and Zero have wrought by being untrustworthy cowards.
We truly need a Ronald Reagan, wise strong, trustworthy.
There wasn't the slightest possibility that SDI ever could have stopped more than a fraction of an attack of thousands of missiles, even if the entire GDP of the US had been spent on it for ten years. A lot of the promoted technologies were essentially fraudulent from scientists just looking to get funded.
In terms of stopping a raid BMD defense MIGHT work against an attack of < 20 missiles from Korea or Iran.
No aspect of the current system has been credibly tested under realistic conditions (where warheads employ decoys and penaids, etc.)
Of course then you get into the issue of why it freaks the Russians out so much, which is more complicated.