Speaking for myself, not for the board: Phabricator is continuously breaking down, and while duct tape is being applied, we are on the wrong side of truth here and this is only getting worse over time. This is an infrastructure issue, which is why the foundation cares, and the point of a long transition is so we can figure this out together.
I’m not aware of credible alternative solutions that are worth considering here, but if there were a specific proposal and it met our general goals then of course it would be considered. The broad goals are something that is hosted infrastructure, that is widely used by the software dev community, that integrates with github based source control and supports the necessary features we as a community are looking for, well documented / easy enough to learn, etc.
I’ve heard and seen surprise from a number of folk along the lines of “LLVM has tons of big companies using LLVM, why can’t we afford to build our own thing (e.g. like Mozilla did)”. The former is true: massive companies use and contribute to LLVM. That said, in practice, no one has stepped up to help with things like this. Perhaps because building something /good/ (not just a demo) is a huge amount of effort.
If you think this is a project /your/ organization should be driving, then talk to your management about it: act instead of whining about other people not acting :-). Similarly, if you or your organization has connections into Github, then please use them to get improvements. This isn’t my area of expertise, but I can’t imagine that “adding comments on lines that aren’t touched by a patch” is missing due to technical reasons, it is probably more about prioritization.
One other thing I’ll add is that this thread is another indication that people who are grumpy with things often speak up, but folks that are happy with things often stay silent. I can speculate why, but I would still love people in favor of the transition to actually say so 
-Chris