I don’t expect Amazon to renege on their licence, in fact I expect them to last a long time. Maybe they’ll outlast me. But I’m not psychic and neither is anyone else. I do not expect it to ever be a problem, but I disapprove of DRM in any case so I’m happier without it.
The amount of paranoid hysteria Amazon provoked by their one and only deleting-a-book incident is a guarantee that they won’t be doing anything like that again.
Now, it may or may not be legal to remove the DRM, but it’s perfectly legal to back-up copies of your Kindle books, and in my opinion it’s foolish not to. In case I’m not being clear enough; you can make as many copies of DRMed ebooks as you like and store them anywhere you like. DRM doesn’t prevent you from making copies, it prevents you from opening and reading backed-up copies except on authorised devices/software.
ETA: I mention the backing-up because even if Amazon did remove a book from your Kindle, they can’t touch backed-up copies of the same book in other locations.