It's a shame that DropBox is killing these two great additions. I got invited to Mailbox a while back during the beta, but after a few months, I gave up on working with a third-party application for my inbox. I needed something that wouldn't constantly bother me, as I get e-mails like crazy from ~7 different sources (work, school, gmail, aol, another gmail, and one more for my work).
I have been using Carousel since it came out. Initially, I was skeptical on differences between that app and DropBox. However, I noticed that it would back-up all the images to a timeline, allow for favoriting images (handy when you need to pull up bus schedules and stuff), and really allowed me to have easy access to images online/offline while taking up pretty much no space on my phone.
Dropbox as a company has overvalued their services provided to customers. Cloud storage is becoming unlimited, cheap/free, and everywhere. From pirate-haven sources like MEGA or file-sharing domains with apps to accompany them, to Google Drive, iCloud, and OneDrive. I'm also in the free-tier and would absolutely pay... For a reasonable price. They don't have any options that work. It's either pay a lot for a lot, or pay a little for a small storage. Much like PlayStation Now, DropBox has really failed to have a good subscription-service payment system.
While I have really enjoyed DropBox, I feel that with time, if they refuse to advance as a company, they will eventually crumble. Much bigger companies are already at DropBox level or will reach DropBox level of services in a short amount of time. For example, iCloud now allows access to images and documents across most devices (even Windows-based devices).
The future of DropBox unfortunately, does not look very bright.