If by whiskey. Sure, the conception of SDI is untenable, but a successful orbital weapons program of some kind isn't. Enough high rolls and they'll have nukes in orbit, pointing down.
Materially not much worse than the existing MAD, but it'll be politically a massive shitfit at minimum. A more effective program might be one to just accident Soviet spacecraft, which by certain means would be nearly indistinguishable from natural incidents.
What do you mean by "if by whiskey"?
And... Space based nuclear weapons aren't SDI. They are a different kind of stupid.
For a start, leaving them in orbit is a significantly worsening of MAD, since it cuts down the decision time from about half an hour to about 5 minutes, making accidental nuclear war far more likely, as people don't have the time to check false positives before they have to authorize a "retaliation" strike.
So putting up orbital nukes is an act of war, no ifs or buts, since the only reason to put them in orbit is in preparation for a strike.
To make it worse, orbital nukes:
a) stand out like a sore thumb against the coldness of space
b) carry limited fuel, so can't dodge for shit
c) can be intercepted and destroyed by vastly less expensive munitions (it's easy to get something high, the hard part about orbiting the Earth is getting enough speed to miss the ground as you fall - an interceptor doesn't care about the ground)
Orbital nukes are vastly more expensive, more vulnerable and less capable than submarine launched missiles.
Kind of depends on what you mean with good I guess, if you mean it achieves a reasonable intercept rate. Then in our future eventually that may well be possible, but it's obviously impossible for 70s or 80s tech.
If you mean, pretty much perfect intercept levels... then yeah, that would be asking for the near impossible. Not like the other side won't work on improving penetration ability after all.
By good I mean "remotely worth building".
Orbit is a terrible place for weapons. There's no-where to hide, there's nothing to dump waste heat into and its expensive to reach.
Also, for SDI to have coverage over Soviet siloes every minute of the day and for enough defenses to be over the USSR to attack a theoretical Soviet strike as it was leaving its siloes, an absolutely ungodly amount of crap needs to be shoved into orbit. Astronomers in such a world would envy our world's astronomers and their problems with Starlink satellites getting in the way of their telescopes.
To make all that worse, to have fast response times and to deal with issues like laser dispersion, the SDI platforms need to be in fairly low orbits.
And there is a serious ammo/consumables problem. Practically speaking, any defense platform is a single-use system. Either you need an energetic chemical reaction to get the high peak power output for things like lasers or particle beam weapons (meaning enormous masses of fuel for even single-use death rays, let alone multi-use death rays), nuclear explosions (for the bomb-pumped x-ray lasers), even one of which being lit off starfish primes the near-Earth environment or racks of expensive guided missiles.
Then there's the problem of hitting an enemy warhead. The easiest thing to do would be to hit the warhead from the ground as the now hypersonic warhead falls towards its target - but it is fairly easy to overwhelm and boosting to intercept a hypersonic target requires tracking and acceleration that is, while so far as I know feasible, unavoidably expensive. In a full WW3 scenario, the best this could do is ensure that some missile silo could survive long enough to execute a second strike, achieving the same results as an SSBN but costing far more resources. Next easiest is to hit the warhead at its apogee in space, which is a short window and is troubled by the fact that even if you can destroy the warhead, you are doing so by expending vastly more resources than the attacker has expended in attacking you, since you need to have space based systems competent enough that they aren't foiled simply by the warheads having a coat of shiny paint, and as we are learning with drone warfare in the modern day, countermeasures that cost more than the thing they counter aren't all that useful. Hardest is to damage the missile boosting the warhead close to the ground, since you have to punch through the atmosphere to hit the missile there, you require quick interception, high power, optics and tracking that the laws of physics frankly don't allow, at least not in combination. Better just to do a saturation bombardment of the silo field as soon as you see the covers start to pop, but that brings us back to the hideous expense of putting a butt load of crud into orbit, all to do a job that a SSBN hanging out in the Arctic ocean could manage at a fraction of the price by launching a nuclear strike on the siloes.
So theoretically, one could just brute force the problem, much as one could theoretically stop every bullet being fired in a battlefield with a big enough anti-bullet machine gun. But the cost of such an area saturation system would be so absurd that any army that attempted to build it would bankrupt themselves long before they reached the battlefield.
Similarly, SDI is not remotely feasible.
Also, please note that I am being extremely charitable and assuming a bunch of problems that real world engineers haven't been able to crack in more than 40 years of work can be solved. I'm not sure that these engineering problems will ever be solved though.
Regards,
fasquardon