I’ve never seen someone underspend a budget and be thanked for it.
Of course, many agencies can quite quickly find places to spend money beyond AdSense :)
The blog post failed to mention that when scaled up there is much more in play than strictly consumer acquisition (e.g. brand awareness) that is just as important when marketing in most B2C verticals.
You might run conversion lift tests as a way to calculate the impact of increased brand awareness, and you could plug the data from these into the method outlined in the post. There you'd be looking for the optimal cost per incremental conversion rather than just optimal cost per conversion.
This is just one idea, hopefully shows that the post's method doesn't have to just use last-click conversion data.
That's far too strong a statement. Plenty of small businesses could be more flexible than this if it made commercial sense.
I’ve never seen someone underspend a budget and be thanked for it.
That's a problem with the businesses you've seen, not the principle of not spending money unnecessarily.
In any case, my experience is that "in the real world" is code for just giving in to systemic inefficiency and purposefully mismanaging your resources.
I've worked agency and in-house, and most of time I've worked without budgets, just trying to maximise volume at a particular CPA. More brands are moving this way, but a few do still just stick to fixed budgets.