SDI was a tremendous cost and drag on defense spending. It actually undermined spending on 600 ship Navy and other programs that were much more relevant to national security (equipment we were likely to use, rather than nuclear weapons or defense systems that were likley never to use because MAD). The R&D on SDI has eventually led to some programs that we take for granted though - Patriot, Aegis, and even the laser point defense system now being fielded by the Navy. 30 years ago, we simply didn’t have the computing power to make those systems operate (always 5-10 years away, not unlike how close we are to getting fusion power).

Now, we do have the ability to make missile defense systems functional - point defense and terminal phase, but not space-based, and certainly not in the launch phase.

But Reagan understood that the fantasy of Star Wars and the spending it would spur would have disastrous effects on a then Soviet economy that was teetering. Still, it wasn’t SDI that toppled the Soviet system, it was a combination of Chernobyl, glastnost, perestroika, and Gorbachev realizing that indiscriminate spending on nukes was a no-win for both the Soviets and the US.