I'm not a CAD expert but I've used some and frankly I've never encountered (or at least noticed) the "deselect with esc only" feature. My opinion is that the universal click-select action has an already consolidated behavior across many different programs and platforms.
I find quite natural clicking nothing to select nothing because you click an object to select it then if you click another, the second is selected and the first unselected (and this is very convenient). If you want to add to (or remove from) the selection you click with a modifier. That's pretty straightforward and I think that many users are already used to this.
I agree that making complex selections can be hard if your fingers and nerves are not firm enough but I also consider that complex selections are usually needed far less than simple ones so the normal select behavior is convenient in much more cases. Also, you can always disrupt your carefully crafted selection just clicking on another object, not only on an empty area, so how would you manage this: making the add/remove to/from selection sticky? This would be a killer in normal usage. A much simpler and less intrusive solution could be making the add/remove modifier persistent, just like caps-lock (and you may use exactly the caps-lock key for it).
I wouldn't sacrifice general usability for such a particular need, or at least I'd make it configurable as it's a matter of personal habit and attitude (that's to say that you could make someone unhappy with the modifications this thread is about).
Luca
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