I don't get this logic. Obviously you are going to consider him the GOAT and refer to his peak as evidence of that just like others do for Pele and Maradona and Zidane and etc.
Pele, Maradona, and Zidane didn't regress at 27 years of age.
By your logic, Ronaldinho should be in discussion of GOAT but he's not because similar to Messi, he regressed at an age he probably should have been at his best.
Messi is still great but he is not the GOAT by a far stretch. Talk about statistics all you want. GOAT is someone with statistics and dies and lives for his team. Someone who is outrun in a World Cup final by a 36 year old is far from the GOAT.