I do not think 9000 further less police officers will make the slightest difference in detering an IS terrorist attack, if they are planning one.
But it will make a difference to YOU if you are burgled, robbed or attacked. That is because firstly, the criminals will know they are less likely to be caught carrying out the crimes. And secondly, they will know they will be less likely to be caught afterwards. That will encourage more criminals, and the ones who already have that career will be active longer before they are apprehended.
The Conservatives are daft. They know we are likely to see an increase in crime and a decrease in people getting caught. They have made a political judgement that that os acceptable. But it is a gamble. They might get away with it, or there might be unforeseen consequences (unforeseen to the Tories, not everyone else).
In the end, policing should be about getting the right balance. The Conservatives say that falling crime figures* are proof that police numbers are too high. I would maintain that the falling crime is proof that the police numbers are just right, or were.