Most manufacturers quote this in relation to service life , 10 years being in ideal environment.
I do tell our clients this, as I am only passing on the manufacturers statement .
You either wish to take up their advice or not its quite simple, what does annoy me is scare tactics ie no insurance cover , so what happens if a detector fails during the period of the inspection visits , and why have 3 visits ?
You could get them to change say 3 or four on each visit so its no a big outlay.
As Antony has detailed , some jobs we have are 15 year old and they work fine , if you are getting false alarms then thats another story .
I don't think anyone is having you over , its the way its been explained to you thats half the battle nowadays.