You need to 'detect' centrifuge building and storage otherwise their breakout time ( = time to get nukes from the time they decide to just ignore the arrangements) is reduced to nothing and there isn't enough time to stop it.

Unfortunately, centrifuges don't have to be loaded with radioactive material before activation, so a large construction program would be undetectable under this scheme (especially given that some research and construction of centrifuges was allowed).

Another problem is actually the long trace detection time: that allows Iran to claim any trace is from the old nuclear program, and not a new one (that claim already happened. It's probably right this time). Since Iran was never asked to account for the old program, it can make the claim for any new site ever. It's not trivial to disambiguate whether a sample is new or old after a certain threshold, these are trace amounts after all. There would be plenty of room for doubt. With no possible proof, there would be no support for action against any new program.

"These things are not terrorism..."

I gather that Iran's support for genocide/mass ethnic cleansing is no biggie, but if you allow that in service of the deal, I wonder what you will allow in the future.