> Obviously if people were only contagious if showing symptoms, the pandemic wouldn't be.
I don't think that is obviously the case at all. It would have to be proved. The dominant vectors of contagion are not yet well understood - we could be dealing with very highly infectious airborne virus, that is capable of spreading widely from a relatively small number of symptomatic and pre-symptomatic super spreaders.
The fact remains that current public health interventions orientate around isolating individuals whether they are symptomatic, asymptomatic or paucisymptomatic. Moreover, to greater and lesser degrees we also isolate the healthy, assuming them to be asymptomatic in an abundance of caution. Considering the negative non-covid related knock on effects of these policies its at least worth investigating isn't it?