What happens if a crew complies but does not read back ATC instructions?
No, ATC would not file a complaint for a single missed read back. In order to file a complaint they'd need to have very good reason to believe that the pilots were willingly breaking communication regulations, and there are so many other valid reasons for why this scenario could happen:


The pilots may have responded but it may not have been received. If two stations talk at the same time the stronger one wins, so they may have read it back but it was lost
The pilots may be dealing with a problem, or have a high workload. Communication is the lowest priority, flying the airplane safely is first, then navigation
The pilots may have a technical problem with the radio
The frequency could be too busy to get a read back in, sometimes the frequency can be so dense with communications you can't get a word in edgewise, so the pilots may comply and eventually just give up


These are just a few reasons from my own personal experience when I haven't read back an instruction or my read back wasn't received. If ATC is worried about it they can always call me back.  

EDIT: The readback is a requirement for some instructions and I am in no way saying it's okay not to. My answer is for the specific scenario given, I think that the FAA would and should bust people who fail to readback in certain circumstances.