For many people it's not even a case of "saving up for a machine," because they might be using donated hardware. For example the OLPC (One Laptop Per Child) project that runs Fedora on low powered machines[1].
Even for people who can save up for new machines and do install Linux on them, that $100 is still an additional expense they wouldn't have needed irrespective of any comparisons to Microsoft nor Apple "taxes".
For these people, particularly the first group, Electron simply isn't an option. So at best developers are depriving themselves of a future growth market but at worst they're excluding deprived sections of society who are already isolated from the IT industry as it is.
That all said, I don't mean any of this as a criticism of yourself nor what you had written. None of what you said it technically nor morally wrong despite the points I've made. I see this as similar to the discussions made about non-English speakers being disadvantaged when learning programming languages.