> Say you vote for a tax increase. Everyone pays. You don't.
That's actually a bad example, because US citizens have to pay US taxes even if they've lived abroad for many years. As a result they also have to have at least a US mailing address and a US phone number since they still have business with the government (Uncle Sam). Also the opposite is true, you would be incentivized to vote for tax cuts instead, because you wouldn't reap the benefits of any social program that isn't tied to Social Security.
> That means you have been a part of the affected population recently enough and long enough to have "skin in the game".
You're take is a little too cynical in my opinion. Americans that move abroad don't magically stop being Americans, and they usually don't stop giving a shit about their country. Also as a superpower, what happens in the US likely still affects their life abroad. More likely though is that people who have lived abroad for many years are actually out of the loop, don't really know what's going on in the US and what's relevant or important, and so wouldn't make good voting decisions.
> billions of people under totalitarian regime sends millions of people to another nation. They instruct them to stay there long enough to become citizens of that nation
This is one of many variations of security issues, another might be foreign adversaries buying up properties or businesses, etc. I'm pretty sure the military is aware of these threats and has ways to prevent them from happening. They probably also have ways of dealing with circumstances that are technically legal but still a threat to national defense, as I'm sure things like this have happened many times over in different ways over the decades, and continues to happen.
> the potential nefarious use of "remote voting" in any nation
A valid concern but a bit too specific and slightly xenophobic. Really the problem is more general than that, as there are all sorts of conflicts of interest when it comes to voting, so an actual comprehensive solution would need to take all of it into account, if it's possible to sort it out.
For example, a poor person would be incentivized to vote for social programs that are targeted to support them economically, because they won't be paying for it in taxes. A rich person would be incentivized to vote for tax cuts for the wealthy. A religious conservative would be incentivized to vote for policies that align with their religion. A progressive would be incentivized to vote for policies that align with their progressive ideology. Etc. And as you can see, it can be complicated if not (currently) impossible, to sort out "valid" democratic participation over invalid participation.