Hell, half of the time (pre-covid) I/we used to end up in conversations with the propietor during these slower times and we'd catch up on how things were going.
Once you find a place you like, it's not like most people are just anonymous singletons coming in and ordering a coffee and taking up a table for a whole day. You keep the place busy during slow periods, other people see the place has custom, and depending on what's there you'll invite friends or other people over or drop by at other times for meals.
On the weekends or during particular busy times, sure. But usually you need lunch or dinner during those times as well, or you'll invite people for those times.
The successful cafes around here got quite skilled at appealing to the different groups and swinging demands throughout the day. The after-work crowd is different from the workday crowd is different from the tourists and the weekenders is different from the locals, and you need to keep all of them happy to keep the place ticking over and remain profitable. It's not just a substitution between all types of customers...
I guarantee the proprietor will be complaining as soon as you have left.
If the place was busy and you were occupying a table without ordering anything, maybe. But if the place was otherwise fairly empty, they probably don't mind very much. It can even be a benefit to have a few tables occupied so that the place doesn't look completely dead.
It depends on the place. There is a place I go to in Berlin sometimes where if you show up, they'll ask you what you want once, and never again, and they'll leave the cup on your table until you re-order through your own initiative.
This is a very nice system that I'm sure they do to attract laptop people during the day (it's otherwise always at least half empty during that time).
There is a sign on the wall that kindly tells people to put away the laptop after 21:00 which is when they get busy. I think it's a fair deal. You get office space for the price of a coffee and they get someone in an otherwise empty café during the day. (Who often order a bit more than just a coffee anyway).
Because if someone complains about you it is because they feel you did them some wrong. You should try to lead your life not making others think you are offending them because society as a whole is better when people are happy and not pissed off.
I guarantee the proprietor will be complaining as soon as you have left.