It's absolutely my stance that the bracket of the population who are currently - almost a year on - still foaming at the mouth with "selfish!" at people daring to visit their own flesh and blood, were likely well-adjusted to lonely, boring, uneventful lives pre-2020.
If you think lonely people are the main ones foaming at the mouth and judging everyone, then I'd question your understanding of social isolation. We're not all curtain twitchers who hate other people, the stats on it show it's largely around life circumstances and health -- and many aren't "well adjusted" to it (hence lonely and not just alone - and why would those happily alone automatically care about what others do?).
Pre-2020, there was a lot of discussion about the epidemic of loneliness and isolation in the UK and similar nations. A lot of it was around the health problems associated with loneliness - some have discussed how lack of social connections has a greater correlation to some medical problems including early death than more notorious things like smoking and obesity. There was a lot of discussion on the distress and pain of loneliness, and there has been quite a bit on how badly its affected people now that even the kind of bumbling social interactions with shop staff and similar has been reduced/cut off.
Whether the same dramatic language will be used again I'm not entirely sure, but I'm someone who has fit in that category - when the person you've seen the most in person in the last year that you don't live with is your dentist, that becomes pretty obvious. I'm not well-adjusted to it, I end up in tears about it regularly. Most of my close family members died within the last 5 years so I'm not sure I'd call that uneventful, but yeah, maybe I'm boring and don't have loved ones beyond my home that want to see me, I fully recognize that I'm not anyone else's priority and when people can pick 1-5 people to see, it's unlikely to be me. Few pick the sickly one who struggles to go for a walk (though I really miss hiking) and anything I could to change my situation would involve meeting with strangers - guess what's kinda hard to do in-person right now and is likely the last thing to come back in a way someone with a mobility disability is likely to be able to access? My life in general hasn't changed much, even with a major workshift it's the same type of digital format, but lockdowns have taken pretty much any option and hope for better, closest I have now is moving to a more accessible home and having a better Groundhogs' day there until in-person groups for meeting people open again.
Stats on a lack of social interaction isn't evidence that everyone in that group enjoys/actively chooses that and there is no reason to think that they're automatically the ones against others doing so (those who have others in the house may enjoy if they could go out more). I personally have plenty of sympathy for others, things are hard for most people no matter if you've loved ones you're seeing, ones you can't see, or none at all, but I think the ones shouting others down as selfish are a complex mix of people with different reasons for it - from those who have been breaking the rules but like a good grandstanding to those who can't see their loved ones until things are far better because they live in other countries that are currently closed, are in institutions that have more control over visitation, or are at significantly higher risk and have been told to continue shielding whether or not they've vaccinated. I don't think it's mostly or mainly us boring fucks no one wants to see even for those of us who pretty much stick to the rules through lifestyle and disability limits.