That's a popular ad-hoc "street" theory (it's easy to come up with those) but formally speaking that idea has never been tested or proven; it's not a part of germ theory. If you look at epidemiological models, they rarely model seasonality for that reason. The models used for COVID for example, didn't have any notion of seasonality in them for a long time and many still don't, because what you just said isn't actually scientifically accepted.
And it would probably run into some practical problems if you tried to nail it down, like the fact that for many people they are office or factory workers and the amount of time they spend outside doesn't change all that radically during summer, certainly not enough to make the difference between explosive spread and total eradication.
"The more people are already infected, the fewer and further between the as-yet-uninfected are, so that seems quite reasonable"
That's why the wave slows down yes, but again, this ad-hoc notion doesn't work. Try it out on paper or code up a quick epi model yourself and then compare against real world case data. The waves always end long before predicted, even when there are still tons of uninfected people available to infect and people wandering around who should easily infect them.
This is one of the reasons why COVID modelling failed so badly. They coded up the exact simple germ theory model you're describing here, and of course it yields a single giant wave whereas what is seen in reality is a long series of small waves that start and end in ways that aren't predicted by the theory.
"See first point above."
See my reply. I can assure you, that the epidemiology of influenza really isn't as simple as "people going outside in summer". Take a look at some of the research papers from the 80s exploring this topic. There are still plenty of people interacting with each other closely indoors even in summer months, yet flu completely disappears. It's not something that ties in any obvious way back to accepted theory.
"Someone always flies in supplies..."
That's not what "totally isolated" means. In the cases in question there was no contact with the outside world whatsoever. In one case, at a British polar base, they mounted a very thorough investigation after an outbreak of cold virus after 17 weeks of total isolation. They checked if any new supply crates had been opened, etc, but no. They couldn't identify anywhere that new viruses might have been introduced to the base.
https://www.cambridge.org/core/journals/epidemiology-and-inf...
One of the more popular sub-theories (though largely ignored by epidemiologists) to try and explain these things is the possibility that many people are continuously infected at a sub-symptomatic level, that the immune system never 100% wipes out the viral infection in these people, just keeps it in check. Then something happens to slightly knock the immune system out of balance for a moment, like a sudden change in temperature, and the infection is able to re-gain a foothold and starts replicating out of control again. So that'd be why people just show up everywhere at once spontaneously infected without any obvious index cases.