Many people are blaming the unusually severe winter bugs on so-called 'immunity debt', the false idea a prolonged period of health makes one more susceptible to infections.
No, you've stated this incorrectly OP. The increase in current illnesses is (according to the immunity debt theory) due to the amount of isolation we've all had over the past couple of years due to lockdowns, mask wearing, social distancing etc. In other words, a lack of challenge to the immune system somehow weakens it so that we are now far more vulnerable than before.
I am a sample of n=1 but this is my two penneth. I've never been overly keen on the immunity debt idea simply because for the past 15 or so years I've been house bound and haven't had the usual seasonal illnesses others get (and I certainly used to get everything going when younger). My DP doesn't bring many things home as his immunity is good, but when he did catch Covid in July 2020, I thought, oh no, this is it for me. I have autoimmunity and so wasn't keen to catch the virus. However I had no symptoms at all. I couldn't believe I just didn't get ill. Now according to the theory, I should definitely have become very ill. Many months later I tested my antibodies and I had definitely been infected by the virus.
This year I did have Covid, as did my DP, and we were both equally ill with it (not too bad but nothing I'd want to repeat tomorrow). I should have been much worse, surely?
I know that's just an example but I don't feel my immune system has had its feet up resting for 15 years, so to speak. In fact, it has been very busy with my autoimmune symptoms. Just because you haven't had a cold doesn't mean your immune system isn't busy with other things. We constantly encounter viruses and bacteria even if we live alone and don't mix, because those tiny things are literally everywhere.