One must evaluate the decline of Taj in this broader context. It is currently maintained by ASI and they have replaced many precious stones abd repaired the Taj as much as they can. Unfortunately they do not always follow the same medival construction methods and do not have adequate funds, so repairs do fail.
Polloution is terrible in Delhi- Agra corridor but one must balance the needs of the poor to earn a living vs. the needs of the tourists to see the Taj in pristine condition. For this reason a blanket ban will not work in practice.
I think the BBC article is one sided, and the complexity of the situation needs to be taken into account before painting a pessimistic picture.
Aga Khan trust has done excellent work in Humayun's tomb and I think it is a better model for moving forward.