In US, is it possible to have a non-tenure position for the whole career As it is very competitive to have tenure, I wonder if there is any case that someone holds a non-tenure position (e.g. associate professor) until the end of his/her career, i.e. until the retirement. If there is, is it a normal practice?
In the US system, it is generally not possible for a person to hold a tenure track but non-tenured position, such as being a tenure-track assistant professor, for longer than a few years (I think the typical tenure clock is 6 years plus extensions for life events like new babies).  After that, the person will either get tenure or else they will have to leave or switch to a different class of position, such as "research scientist."

Research scientists or non-tenure-track professors, however, can easily have a full and comfortable career with nothing even vaguely like tenure involved, all the way up until they retire, go emeritus, or succumb to old age.  This can happen both in traditional academia (see, for example, Dave Clark, or non-tenure professors at medical schools) or in alternate academic settings such as national labs and research companies.