This does seem to make sense from a purely rational perspective, as herd immunity to many viruses is obviously how we've got this far as a species - but there are plenty of viruses out there, like HIV, that we've never acquired immunity to, nor developed a vaccine for. Sure, there are drugs to manage the symptoms once you've got it, but you can't guarantee you won't get it. We may acquire immunity to HIV eventually, but then it's quite difficult to catch HIV, relatively, compared to this coronavirus, so maybe not. There's also the emotional/spiritual and practical impact of many, many people dying very rapidly, which most of us are not used to, and may overpower the efforts of many individuals to 'rationalise' the situation.
The big problem is our population spread - the older age groups, at least in the developed world, are huge, and so the likely outcome of his scenario is massive death tools in that age group, if they have the 'Group A' weaknesses. They are also a comparatively wealthy generation compared to the ones below them, though clearly not evenly spread in some areas, so large death tolls may have some economic re-distributive effects not mentioned here. Like big houses becoming available.
One other very important issue here that I haven't heard discussed anywhere yet is that of sex. As in, when will the general population get to do it again? At the moment, the only people who can practically, safely (and, by implication at present, quasi-legally) have sex are those who already live together and have clearly also committed to enduring the lockdown together (haha - that wasn't in the wedding vows!). No-one else is 'allowed' to meet, and if they do, certainly not close enough to have sex with each other. If those people already living together are heterosexual and contemplating reproducing together in the near future, they may be reconsidering their plans. Everyone else can forget about it.
Obviously a proportion of babies born under regular circumstances will have been unplanned, or possibly even as a result of a 'one-off' encounter with a non-permanent partner or total stranger - it does happen. Or it did. So if we have a situation with an increasing population of older people and a rapidly decreasing stock of younger people to replace them, which has been happening even before the virus, this will make matters a lot worse. There may well be a 'baby boom' after all this is over, as all the younger people start having sex again, but that will be years down the line - for one thing, folks may be far more afraid of a one-night stand that they used to be. Or they may be far better with contraception in future, to avoid catching the new virus now - though ironically (I'm guessing!) kissing would be far more likely to give you Covid-19 than penetration, with or without a condom.
It's an interesting, but rather difficult conundrum. The author seems to to be semi-advocating letting them at it, though preventing an imminent population shortage isn't explicitly stated as one of the objectives. Maybe reading between the lines it is.
