As I clarified previously, my conclusion is based partially on his PSG form but mostly by looking at how the other big teams play and how he might fit in. Same way I don't need to see Busi at Liverpool or Bayern to surmise that he would probably struggle there.
There is a fatal flaw in this reasoning, especially using Busquets' example:
Busquets's recent seasons don't include a Ballon D'Or level season in 18-19 (51 goals and 20 assists in 50 games, 12 in CL) and a Ballon D'Or contending season only a couple months ago (2021, ~ 60 goals + assists, Copa America best player etc.) Busquets has been trash and one of the main reasons behind our CL woes since 2016-2017.
So while the extrapolation may be plausible in Busquet's case (I would agree it is), Messi's most recent showings aren't at trash level to justify such an extrapolation.
And Busquets was never even remotely close to Messi in the all-time standings. Usually, GOAT tiers find a way to make it work.