I don't know why anyone would ever want to do that... there is a perceived notion that it's cheaper... but only if you are trying to do basic things.... if you have complex needs you will be nickel and dimed to death.
Our org is looking at switching (because they think it will be cheaper)... even cutting out massive amounts of things that we can do today the assessment is still more expensive (since we would have to create rollups of some of our most combined sites and all the server calls in those rollups cost .5 of a server call, then we also have to try and replicate Activity Map - which GA cannot properly do, which means click tracking on every link, increasing the number of server calls). GA has no concept of a segment repository, so short of duplicating an existing report so you can have a copy of a segment that you built, you will have to recreate your segments for every report.
Most companies I have talked to who only have GA360 do 99% of their reports in Big Query / GCP because the interface of GA4 is so poor they can't do what they need (spending hours on SQL queries to pull reports, instead of the 10-30 minutes it would take in Adobe Workspace)... many of these companies are switching from GA to Adobe.
As someone who handles the Adobe Analytics and (free) GA for our company.. I hate working in GA.. and when I have to for some of the smaller sites that only have GA I end up being frustrated and spend far longer to pull simple reports.
