How would you address the head-trainer of your footbalteam; with "coach" or "trainer" or with his first name?
How about your physician, would you say doctor or use the first name?
Or the teacher at the university; "professor" or his first name?
Or your priest; "father" or his first name?
Your instructor in fencing; "maitre" or his first name?
How about your father? "Dad" or first name?
How about the queen of England? Would you call her Liz?
How would you address the president of the USA? Mr. President?
How would you address the prime minister of a foreign country?
Or the dalai lama? Or the pope? Would you use "his holiness" or call him by his first name?
It depends on how well I'd know the person and on what they prefer.
With people I don't know well and people older than me, I would use Mr. / Mrs. (+ their last name) and if etiquette requires a more specific honorific, I'll use that: in the karate dojo it was explained that different ranks had different titles, 3 dan and 4 dan was called sensei (sempai for 2 kyu to 2 dan and shihan for 5 dan and up). Outside class, we'd adress them by their first name.
If I know people a little better and if they want to be addressed by their first name, I'll use that.
It just happens to be that the Dutch aikido teachers that I know, prefer first name only.