Work never ends. No matter how much you get done there will always be more. I see a lot of colleagues burn out because they think their extra effort will be noticed. Most managers appriciate it but do not promote their employees.
What I instead do when people overwork is I give them extra paid time off. Stayed in the office for a couple of extra hours yesterday helping a customer? Great, please make sure to take Friday afternoon off. You had to work extra last week to meet a deadline? Apologies, that was my fault for not managing your workload properly and distributing it better (and also not setting a reasonable deadline). Second, again, please take the equivalent amount of time off.
The one is having fixed working hours and, as in the parent comment, compensating with extra leave if those hours are overstepped.
The other is working overtime whenever there is extra work that has to get done.
> obsession with 9-5 and whatever else it takes
You mean an obsession with "working 9-5 ALONG WITH whatever else it takes", not an obsession with "9-5" and a separate obsession with "whatever it takes".
If you are looking longer term, avoiding burnout and employee satisfaction will be a net positive. If you are looking short-term (12 months at most, but 3 months is more of a burnout limit), you may benefit from overworking people.
Work isn't about getting things done, but maintaining a consistent throughput.
Tripurarahasyam (from wiki: The Tripura Rahasya (Devanagari: त्रिपुरा रहस्य, Tripurā Rahasya) meaning The Mystery beyond the Trinity, is an ancient literary work in Sanskrit believed to have been narrated by Dattatreya to Parashurama. It is an ancient prime text which is one of the treatises on Advaita school of classical Indian Metaphysics.)