I’m guessing their “momentum” wasn’t on the path of building a sustainable business, particularly given the existence of Microsoft. From a market position standpoint, Paper could have become what Notion is now. Notion even released at around the same time as Paper.

Was it that Notion from early on supported Markdown compatibility, which is a distinguishing feature when selling to software startups? Was Paper too aggressively coupled to Dropbox storage, when Google, Microsoft, and Box already had a large share of that market?

Paper still exists - Drew refused to kill it. Been heavily underfunded for years and saddled with a codebase that assumed it would be >50 people forever, so still digging out of that hole - hence the apparent freeze on feature development for multiple years and separate mobile/desktop apps disappearing.

Yes, the photos one was Carousel. I'm still sad about that, in some ways it was better than even what Google Photos offers now. The big problem there was Dropbox chicken'ed out about picking a fight with Google Photos - since Google were giving away for free(ish) a comparable product to the whole Android install base, doing the same freemium model as always would've been a challenge with heavy competition for the free users.

Mailbox... was a botched acquisition. Ideologically they aligned, but no real plan for how to keep momentum and turn it into making money, and no real story about how Mailbox was better as part of Dropbox than separate. Even under the hood the infrastructure was all separate (except eventually for login).

A big missing piece of the story: Dropbox tried to be a multi-product company long before building the infrastructure for that. Which I think self-sabotaged many of these new products.

Paper still exists, though has definitely spent quite a while stagnant due to underinvestment.