@kagixa,
I doubt that Purism is going to decide to switch to Arch. Guido Gunther and Matthias Klumpp are Debian developers, and the former CTO Zlatan Todoric was one as well, whereas Purism doesn’t have the same level of expertise with Arch.
I think that it is more realistic for you to push for the creation of a Purism community wiki, where the Arch community can document how to use Arch on Librem hardware and we can pester Matt Devillier for how to do things like update the firmware when using Arch. I think that Purism needs a community wiki so people who use other distros don’t feel left out.
Right now the Debian Testing kernel (5.10.46) is significantly behind the Arch kernel (5.13.9), but that only happens during a freeze before a stable release. Most of the time, there is only a month or two of difference between the kernels, and the thing that really matters for Purism is Coreboot support, which is mostly determined by the cooperation between Intel and Google for Chromebooks, so I really doubt the speed of kernel updates matters that much.
Interesting that someone uploaded ProcessMaker to AUR, but that package is 4 years out of date, and anyone installing it will want version 3.6.5, not version 3.2.1. The issue, however, is that ProcessMaker Inc. refuses to support Arch as a platform, whereas ProcessMaker Inc. does have a few clients that use Debian, although most are using CentOS. The creators of commercial software generally don’t support rolling-release distros because they are too much trouble for them to maintain, and many companies aren’t going to take a chance with an unsupported platform.
I haven’t installed Manjaro, so I can’t really comment on it, but I think it is unlikely that Purism will base PureOS on another company’s distro, for the same reason that Purism chose Debian over Ubuntu.
Arch has fantastic documentation for expert users, but it can be overwhelming for new Linux users in my experience, and when they do a Google search for how to solve a random Linux problem, they are more likely to encounter instructions for the Debian family than the Arch family.