The harm is implicit above: people who don't get the top grade don't carry on with the subject, not realising that the difference between top grade and next to top grade is meaningless for their ability to carry on with higher level study.
In particular, the new top grade harms diversity as students from non-selective schools are pushed even more away from subjects such as Further Maths into soft subjects which won't get them into top universities.
Moreover, risk averse students from all backgrounds are put off taking the A level by not getting the top grade. Many females are risk averse so your new grade pushes the percentage taking e.g. Further Maths or Physics even more down than it currently is.
Currently the extension maths GCSEs don't separate according to ability for higher level study; performance is strongly correlated with teaching time. The existence of extension maths GCSEs puts off certain demographic groups from studying FM: students assume that if they haven't studied the extension GCSEs then they can't do FM.
Finally, no university department currently uses grades in Stats/Further Maths/Additional Maths GCSE in any way for their selection because of above concerns: grades reflect the teaching time and quality of year 11 teaching as much as students innate ability. It would be ludicrous to rule out a student for not having a top grade extension maths GCSE when their sixth form tutors report they are scoring 90%+ on mock A level papers.
GCSEs are simply not a good indicator (on their own) of students performance at higher level. At most they can only be used as a small percentage of selection. Even those places which select for interview partly on GCSEs don't tend to distinguish between e.g. 6 A stars versus 10 A stars.