This must seem eminent good sense to most City fans but unfortunately cost control measures will not be agreed on here but in the much less dispassionate atmosphere of PL meetings and votes. The sad fact is that not all PL clubs agree on what is important to sustainability or even on which club's sustainability is important. I believe several clubs don't even see why they can't simply re-enact the old rules! They are unlikely to vote to recognise low or no interest loans as ATP's. Then there is the PL which seems quite happy abusing its dominant position and has hardly been a shining example of an even handed governing body.
But the real nightmare scenario might grow out of your last sentence - "the top clubs have to meet UEFA FFP anyway". Let us suppose that rules which satisfy the requirements of the PL and our laws are introduced. We are then to assume that these meet the requirements of UEFA's FFP and that UEFA's FFP meets the requirements of EU competition law, which is identical to ours. This is at least open to doubt and the road could be open to years of legal wrangling, to the detriment of the game on a continental scale.
I agree with your sentiments, but "biting the bullet" doesn't seem tickle the taste buds of many clubs, the PL or UEFA.