1. Work is awful and people hate doing the work they get assigned
2. Therefore, people will try their best to do as little of it as possible
3. Thus, if there's no deadline, or the deadline is very lenient, then people will mess around to fill the time -> Parkinson's Law!
Turns out that if you stop the military carrot & stick model of work, and instead try to actually be a place where people want to do great work, then this dynamic changes completely. We do it like this:
1. Make sure every employee does some amount of customer support. This makes everybody feel that real people are facing real problems that need real solutions, fast.
2. Give people a lot of freedom in what they work on, in what order, so long as it fits broad company objectives. If a customer reports a bug and it "nerd snipes" you completely, go for it!
3. Teach people scoping and iterative delivery. Stuff like "ship it when it's better than what's live now" (vs "ship it when it's perfect"). Many engineers like to polish things forever until it's perfect (this is the less cynical interpretation of Parkinson's law) but you can just coach people in this, takes a few months max.
I can honestly say that our dev team is the most productive team I've ever been a part of, and we do this without deadlines, without yelling managers who turn your estimations into promises, without overtime, and anything else like that. Parkinson's Law doesn't need to hold. It's a consequence of a shitty low-trust working culture.
Admittedly I've no idea whether our approach scales to larger companies - we're 15 people. Also we hire explicitly for people who are able to handle this kind of freedom. But it works excellently for us.