A few comments/questions:
1) Which particular ARM subarchitectures are being proposed for promotion? Currently there is:
* armv5tel * armv7l * armv7hl
and soon 64-bit ARM.
2) The question about slowing down builds has a misleading answer. It focuses on mass rebuild, however builds for individual packages _will_ slow down because more builders doesn't really help on a per-package basis. E.g. if you're building for i386, x86_64, arm5, arm7, and arm64 (arm8?) then you'll now be waiting for 3 additional arch builds to complete on a per-package basis. Even with enterprise class ARM servers running at 1-1.6GHz, the individual ARM builds are likely going to be slower than the x86 builds. I would suggest either changing the answer to reflect that, or provide data (once available) showing that build times for the ARM builds are comparable to x86. This is important because one of the main detractions of PowerPC was the much slower per-package build time that held everything up.
3) I would suggest adding a hypothetical question along the lines of "If an ARM build fails for my package, does it fail the entire build until it is fixed when ARM is a primary arch?" The answer should of course be yes.