One prime example I can give to the nuance is your statement about variants. You seem to be under the false impression that quarantines would have resulted in fewer variants around today. The reality is quarantines can in fact increase the number of variants due to the conditions needed for new variants to become established in the population, which is assisted by small isolated groups of people more than large interconnected populations where there is no isolation.
It again comes down to the nuance of how new variants come into existence and become established, its not as simple as people think where it just happens spontaneously and then it exists. Variants usually require multiple mutations occurring in a series and need to become established so they must locally out compete the dominant strain and reach critical numbers before being released to the general population.
Keep in mind this is very different than other types of viruses, where recombination can occur in a single step.