Regarding this issue, to coat or not to coat and its longevity, in my case it depends on the customer. Usually, my clientele wants something that lasts longer and gives an extra protection from bird droppings and small scratches. My usual customers wants me to polish their car and apply something that add some sort of protection so they can carry on with their lives without thinking about polishing again for at least one year. And to be able to withstand cheap car washes.
They always ask me 'how long will it last' and I say 'it depends'. It depends if you leave you car on the parking lot, uncovered, when you work, if the parking lot is covered or not, if you drive along dusty roads often, if you travel much, how often do you wash, where do you wash, who washes it, how, will you be coming back here to wash it, there are so much variables about each customers car usage that makes impossible to guesstimate the longevity. I always say something in between 4 months to 2 years. I know it is not very precise,

, but what else can I say?
I'm slowly moving towards coating every car I work with, but there is the problem with the cost. The application is a bit harder than sealant, but when you get used working with it, it becomes easier and faster.
My wife's car is a typical example. I did a wonderful job some three months ago, topped with Menzerna PL and the another layer of Nanolex Premium sealant, and then some Gtechniq liquid crystal everytime I washed. But she parks her car under the trees at work, and the zillions birds I have in my country did their job, sealant is not enough to withstand its acidic mess. I will coat it. The same with my car, birds and thin clearcoat. I coated friday.
I even think that Si based coatings, and its variations in formulation, are better than ceramic. I would love to try the opti-coat, but the price per ml is unbearable. I need 2 liters of coating, not some 30 to 50ml.
Right now I have 400 ml of Si based coating, and I'm always looking to try/buy new ones, especially the japanese coatings with strange english names,

. A sedan takes some 18 to 25ml to coat it, so, let's say, one liter is around 50 cars.
Therefore, coated cars have to be more expensive. If the customer doesn't want to pay nor can't afford I apply sealant anyway. No car leaves my shop unprotected.
But everyday you go to work and open the shop is different, you are always learning, and every car that shows up with its own budget is also different.
These are my thoughts for the time being, based on what I've been experiencing lately, I can always change my mind if facts prove I'm wrong, only a stupid person has a closed mindset.
What I try to achieve is a customer leaving my shop happy with the results, taking some extra cards and flyers to give to his friends, after paying me what we agreed upon.
Please forgive my english, it takes me a lot of effort to write it, but I try to detail my english the best I can as I do with my cars. My long two cents.