People seem to think that there's an effect where the individual immune system gets weaker with disuse. AFAICT this isn't the case. The "debt" here is the same debt you incur if you have a Windows PC you only boot up every few months: You'll be slammed with updates when you turn it on, but you aren't getting any more updates than before, in fact you may even get fewer as point upgrades are skipped (e.g. someone who avoids Covid Alpha-Delta but gets Omicron). But since they are all coming quickly it feels extreme.

As pointed out elsewhere in the thread, RSV is more dangerous to younger children, so delaying infection is probably advantageous on a population level. I don't think there is any evidence that the severity of any of these diseases is higher when amortized over the years we had very low incidences, we are just seeing more disease total in a short period of time as restrictions are lifted.