Given that all of this tech is already being used inside Twitter, I feel it is unethical to discuss any non-public info (like discussions with cloud vendors on cost etc.) in public. The engineers have been kicked out, the tech still belongs to Twitter. This is not "decimation" of the engineering, just of engineers' egos. Someone else will do the job, the show will go on.
This is misleading. Likely, those 50 engineers were standing on the shoulders of the giants, i.e. those open source maintainers who build wonderful value for everyone out there.
Discarding the "full stack in a box" idea, in general, just because of past experience with a poorly implemented vagrant setup seems naive. A good "full stack in a box" implementation (docker-compose, swarm, kubernetes etc.) can be a useful tool in testing/developing microservices.