In my opinion the biggest mistake Purism made is to try to build their OS from the ground up, when acceptable and proven alternatives existed already. This has repercussions even now, when users are justifiably annoyed at a mass produced phone with a non functional camera app.

The Sailfish OS and Mer communities have had a working plumbing and porcelain stack for years (granted Sailfish's UI is not fully opened source) but Purism insisted on bootstrapping their own instead of building on those. I feel like they could have forced Jolla's hand to open source the last bits of their proprietary UI and everybody would have been happier.