I think the time taken has less to do with the size of the rewrite and more to do with highlighting Microsoft's level of interest (from a business perspective, not an ideological one) in working on the control panel. E.g. in the same time period Microsoft has migrated and rewritten their enterprise chat platform multiple times across different architectures, frontends, and backends. That's not a point that large rewrites always only take a couple of years, it's a point that they were really interested in evolving their enterprise chat offering.
You start to do a rewrite and then discover that the old design was the right one. But then you double down on those because "metrics" which oddly favors the team/company interests.
