The challenge system would make sense and be fair if it was available on all courts. Since it is not it caters to the tennis aristocracy it should be available to all or none, not the elite.
That all is not equal goes beyond Hawkeye. How about line umpires for example?
ATP match...7 line umpires
Qualifying..5 line umpires
Main draw 5 line umpires (until the semis then 7)
Qualifying.. 3 line umpires
And yes, there are tournaments without any line umpires at all.
The difference here is that within the same tournament, and even in the same round, circumstances differ. That seems a real problem.