I think it's absolutely going to disenfranchise some people, and for some that's a welcome side effect, but I've seen these trends with the post office long enough to believe that's just a secondary effect.
There's been an effort for years to make the post office more profitable by centralizing things more and more and that means downsizing local offices and centralizing processing. That means nothing gets processed until it gets to a regional facility.
I think the issue is not that they are doing this specifically to disenfranchise voters so much as they don't care that it does.