Do helmets go out of style and get discontinued by manufacturers?
In most bike shops, if they have a helmet in stock, they will sell it even if it's last year's model, rather than ship it off to a remainder seller or some such.  But, of course, if they sell the helmet, and the model changes next year, they will not get more of the old ones.

I would guess that helmet manufacturers turn over their product line about every three years, doing a little bit each year.  There's no good way to know if this year's model will be around next year -- it might or it might not.

So you figure the odds.