It hasn't been debunked. What hasn't been proven is if quarantines like we did actually work. The varying levels of lockdown worldwide are not showing any correlation to icu utilization or mortality.
And we didn't run out of ICU beds in NY, NJ, and Connecticut even after we actively took measures to infect the most vulnerable population most likely to need ICU. It is very possible that the net affect of NYs lockdown rules in totality increased the use of ICU and increased death. It protected low risk populations but exposed high risk populations.
For your theory to be true it would mean that it doesn't matter if we actually cure people or not, the people that would have died will die and the people that didn't die could have just 'walked it off'. Which is absurd. The mortality rate is low because we can treat people, but the moment you let the virus spread then you lose the ability to treat everyone and the death rate rises.
And again, each and every single country that tried your herd immunity theory ended up having to go to lockdown, and the ones that didn't is Brazil who is currently having like 300k active cases with 20k deaths, so around 6% death rate.
Also why do you say quarantines have not proven to be effective when countries like Italy and Spain have seen a pretty relevant decrease in cases (Italy started lock down on a 6k ish new cases per day if I remember correctly, they have now between 800 and 600 a day) since they started lock downs and quarantines? And they also saw the decrease of mortality when the burden on the hospital system was alleviated.
And also why are you saying that there's no correlation between icu beds available and mortality when Spain was literally digging mass Graves in the highest peak period, when they couldn't give an icu bed to everyone in need?
Last but not least, NY didn't run out of icu beds because they increased their capacity fearing the worst case scenario,it's not really an obscure info it was all around the internet, normal NY capacity would have never sustained that impact, no country or state has enough icu beds to go for the herd immunity strategy...
If those were infinite and you didn't care
at all about a lot of people having to pass several weeks in intensive care (so the economy would still take a hit regardless) and suffer lung damage that might debilitated them for the rest of their lives, then you might try to go for the herd immunity strategy.
But that is not the world we live in.