A few reasons:

1. Stateful systems (databases, message brokers) are hard to switch back-and-forth; you often want to migrate each one as few times as possible.

2. If something goes sideways -- especially performance-wise -- it can be hard to tell the reason if everything changed.

3. It takes a long time (months/years) to complete the migration. By doing it incrementally, you can reap the advantages of the new infra, and avoid maintaining two things.

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All that said, GitHub is doing something wrong.