The real problem here is the leadership deficit. The manager got pushback on the estimates, and instead of explaining the reasoning and bargaining on scope, he caved and kicked the can down the road by letting the estimates crumble. Yes, estimating is hard, and you are probably going to be wrong, but once you have your finger on the scale to get a desired result, you can't blame that on the difficulty of estimating. Just deplorable leadership.
Because it feels good to complete work and I would like to move on to other features. How long can you work on improving a single piece of code before getting bored?
When I pick up a feature, I know roughly how long it will take and I will try to finish it within that time. I don't need an arbitary deadline from management.
You've formed your own arbitrary deadline right there, because what I was trying to say with my admittedly brief and snarky comment is that a feature being "done" is subjective.
You may have terrific business sense, and more importantly, one that is aligned with your stakeholders. That doesn't mean any other developer does. Agreeing on a deadline in many cases of a healthy team practice.