You could do it that way, but I don't have the need, so my way leads to faster dev cycle: https://news.ycombinator.com/item?id=29566574

Doing as you say requires two deployment for each change with a pause in between, and likely two separate merges (does it?). This reduces the deployment velocity. I might have to do as you describe when I start scaling out - there is no hope of updating many servers in sync so might as well embrace multi-version.