(I'm not sure this is a fair comparison, and am not aware of other possible changes to GHC during those years, and bear in mind that I'm a highly biased party.)
Can you point at a project which made a switch like this and saw an actual significant change in contributions or contributors afterward?
For example, GHC moved from Phabricator to GitLab in December 2018, citing ease of contribution as a major benefit:
https://twitter.com/bgamari/status/1069047550727069696
As of April 2020 (about a year after the transition), the GHC year-over-year contributor numbers didn't appear to change:
https://twitter.com/evanpriestley/status/1245817419441926144
(I'm not sure this is a fair comparison, and am not aware of other possible changes to GHC during those years, and bear in mind that I'm a highly biased party.)
Can you point at a project which made a switch like this and saw an actual significant change in contributions or contributors afterward?