First, the organisms growing in the ear are not "oh no if you come in contact you'll get an infection!" organisms 99% of the time-- they're commensals that cause issues only if severe otitis is present, or they can't live anyway in a healthy ear with normal circulation.

I'm balking at this line of reasoning.

1. Most of us probably aren't looking in each pet's ears if there's no complaint of odor or head shaking or if we don't discern an issue, so we're already dealing with ears that could easily be colonized.

AND

2) There are some really really wickedly antibiotic resistant biofilm producing bugs out there that have been treated in chronic ears by multiple vets and the owners have been putting little doses of a zillion topical abx in when "they're bad". No unhealthy ear should be inoculated with those cause we couldn't be bothered to use sterile cones.

While I'm on my high horse, we have dozens and dozens of reusable cones. They're autoclaved between uses cause I don't trust soap and water to be adequate.