Now obviously this would not result in a nuclear weapon, assuming the country is fractured and unstable. But whoever ends up controlling the fordow site now just needs to do a bit of excavation and we are looking at material to make dozens of dirty bombs, potentially falling into the hands of a group as extreme as ISIS. How is burying half a ton of nuclear material and destabilizing the country at all going to lead to safety for the world?
Others gave good reasons but I'll underline most extremist groups are western proxies and even if they made a dirty bomb out of this stuff the damage to the west would be minimal and would be a welcomed excuse for an even more intrusive police state and powers for the nat-sec state as well as control of people and rollbacks of limited freedoms.
Besides that though, the west couldn't invade and hold Iran. But in your hypothetical scenario where it devolves into civil war, they could do one of two things IF somehow they decided such a buried stockpile was a problem. They could 1) hang back and sponsor various groups, maneuver pliant leaders into place for the various factions so no matter who comes out on top controlling that part of Iran they'll have access to move in machinery to excavate and remove it. 2) Just monitor the site and air strike anyone who tries to move in heavy equipment to excavate. They could also heavily mine it, potentially from the air or via special forces or proxies thus acting as a further deterrent.
For the west controlling Iran or at least removing it as an anti-imperialist power is worth an awful lot of risk, far more than would be incurred by such a scenario you outline. Most likely if they did overthrow Iran and a Syria situation developed, one of the resulting factions would agree to sell access to the material and/or the ability to remove it in exchange for things like food aid, weapons, aid, etc.
It's just 576 but with some black-boxing to reduce it down to 568 in this case a loss of 4 pixels on both top and bottom. So it's an aspect-ratio thing.