The star ruck will play for Sydney - his third club in as many years - next season after his partnership with Max Gawn didn't work out at Melbourne.
Grundy exited the Magpies at the end of 2022 due to pressure on the club's salary cap from the mammoth seven-year deal he signed ahead of the 2020 season.
Buckley, who was coaching Collingwood when Grundy re-committed long-term, concedes the Magpies would have looked "mad" for letting a dual All-Australian go, but insists the move should have been made.
"Hindsight is a wonderful thing, but it would have been better for the club, and it would have been better for Brodie (to) move in a different direction," Buckley told SEN on Monday.
"'Brodes' gets back to Adelaide, he's happy at the Adelaide Crows, who knows what would have happened over that side."
Buckley said there were "internal conversations" at Collingwood about letting Grundy leave.
"Apparently we found a way to keep him (financially) ... which has since proven to be a rod for the club's back, and for Brodie's," Buckley said.
"I'm rapt that (he has found another club in Sydney).
"Brodie has to accept (it), he put his hand up, his manager drove a hard bargain.
"(Grundy) has struggled through the last three years and he had basically been unwanted at Collingwood and for the reasons being part of the TPP (total player payments)."
Taylor Adams (pictured) will link up with his former Magpies teammate Brodie Grundy at the Swans.
The 29-year-old will reunite with Taylor Adams at the Swans after his former Collingwood teammate requested a trade just days after missing the Magpies' premiership victory with injury.
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