You also have the option of building your own battery pack for these, or to disassemble an existing pack and replacing the cells. How difficult that is depends on the manufacturer, but from a quick look at the Neato packs I see it looks relatively trivial.
I'm in the process of replacing the battery in my old robot vacuum right now, and for the amount of time spent doing that, I could buy a whole new vacuum. It's a pity, because the battery itself can be replaced just by plugging it in, but all the batteries I could find were AliExpress fakes, so my two options are either "more e-waste" or "spend $500 in time doing it myself".
Some of us enjoy fixing things like these over and above the hypothetically fungible billable hour, and whether you can substitute some time in an evening with a billable hour or three is highly dependent on your employment situation.
But in this case the relevant cost under discussion isn't that of a replacement vacuum cleaner, but what value you assign to your house not burning down due to a crappy 18650 cell, or the anxiety of worrying that that'll happen.
