The "gatekeeping" back in the days before ebooks was infinitely worse than it is now. These days? Basically anyone who can fill out a webform can publish a kindle book. And other stores aren't much harder. And those ebooks can be sold indefinitely.
Contrast that with needing to find a publisher who is willing to allocate some of their limited production time to you. And then hope that Borders et al are willing to put you on the shelf. And then realize that you are never getting another penny for that book because the first MMPB run ran out and you aren't getting a second because you didn't sell enough to justify it.
Amazon will come into your house to take your digital copies of books you paid for (e.g. when they did that with 1984). No reason to think they wouldn't take physical books after they've violated your digital sovereignty - it is only a question of if that were to ever become a viable option for them.