More complicated than you might think at first glance.
It's certainly not illegal for a user to download something they already paid for from an alternate source. But for the primary source to stop hosting it doesn't instantly make it public domain - for someone like me to upload it on a public link wouldn't be a squeaky clean move.
Something certainly happened here. I don't have any insider info, but I suppose the awful backer portal site they were using charged them substantially enough for them to cancel service after a while. Or maybe that provider simply folded, wouldn't be the first file hosting site to do so. Could be a lot of reasons the backer portal went away.
A lot of the backers who are screaming for their entitlement now might be the same kind of people who would complain about an official public upload: "We paid extra for this, now everyone gets it free?!" This isn't an argument, just an observation of a complicating factor.
Notable: inXile's community coordinator, the front line guy to deal with a customer satisfaction issue like this, just left the company. Like, this week. If anybody new was hired for that role, I didn't hear about it.
It's weird to me that anyone would back a digital download project, then wait three years after release to actually download it, then complain when the download was no longer available. Websites go down, companies change hands, nothing on the internet lasts fucking forever. Check your email, get your shit.
Honestly, I think these people are only complaining because they heard it wasn't available any more. The majority of them were never going to actually download it in the first place.
All that said, I agree that inXile should just put the extra stuff up on Mega and let it fly free.
Lastly, I wonder if Microsoft would be assholes about it if they did that. After all, they own it all now, don't they?