We've already seen a drop in As in the current system in English Language (for two years running IIRC)*
2013: 3.3%
2014: 3.6%
2014: 3.1%
(cf. 1994 1.8%, 1999 2.5%: it was below 3% from 1994 to 2000).
It did peak at 4.7% in 2011, but I think the reasons behind that have been pretty well explored.
I'm in two minds about this (and my hands are dirty, because both my children got twelve Astar, the younger under the newer all-terminal system, and with 90%+ UMS in many of them would would imply they would have got a substantial number of 9s under the new system). On the one hand, I can just about see that there is some argument for extending and delineating the very able. On the other hand, GCSE work is usually of a pretty basic standard so I'm not certain that being able to get 99% rather than 95% on a GCSE exam proves a huge amount other than diligence, and I don't see that the GCSE system exists for, or should even pay any attention to, the admission requirements of a tiny minority of courses at a tiny minority of universities.
Until we decide what GCSEs are for we're all potentially arguing at cross-purposes.
If, for example, they're purely about university admission, then we should go back to A Level circa 1980 and norm-reference, solving the "fine distinctions at the top" by just publishing centiles. Anyone numerate would realise it has error bars a mile wide and the distinction between 97th centile and 99th centile would be meaningless, but university admission is done by smart people who can deal with that.
If they're criteria referenced then at GCSE the argument that the distinctions at the top are meaningless are even stronger (it would be like trying to tell Lewis Hamilton from a journeyman DTM driver by getting them to take a UK driving test and seeing how differently they did) but criteria referencing is what we need for "can you do an A Level in this?" or "are you literate enough to work in my company?", for which norm-referencing is hopeless.
As no-one will answer the question of the purpose for which GCSE is most intended (they appear to be the spiritual inheritors of school certificate, but in a world where study is largely expected to continue to 18) debates about its marking and grading are a bit pointless.