If certain common cold viruses do bestow a level of resistance to COVID19, that might be more of an argument for spreading them.
I wonder if anyone is doing any serious research along these lines !
There have been a couple of papers implying likely T-cell immunity for those who have had previous coronaviruses - so possibly 40-60% of the population would already be immune to, or only have a very mild version of, the current nastier virus. (An ancient equivalent would be cowpox/smallpox).
On the wider point, having a healthy and well-functioning immune system requires regular exposure to nasties. Trying to live in an entirely sterile environment (a) won't work and (b) will probably do more harm than good.