Good question, that was an option. The main motivating factor here was that vacuums were beginning to take dangerously long. O(weeks) to complete, independent of the load on the database. While migrating spaces in segments would have reduced the number of records future vacuums need to scan, we were already running against the clock to complete one vacuum prior to TXID wraparound[0]. To kick off replication for specific spaces we would have needed to write our shard key to all data owned by those spaces. That would further contribute to TXID growth, and was not something we were comfortable doing.

At the end of the day, this is something we could have explored in more depth, but we were ultimately comfortable with the risk tradeoff of migrating all users at once vs. the consequences of depending on the monolith for longer, largely thanks to the effort we put into validating our migration strategy.

[0] https://blog.sentry.io/2015/07/23/transaction-id-wraparound-...