The only problem with using LIFO as the sole method, as far as I can make out, is that it can inadvertently lead to age discrimination and it is that, not LIFO itself, which is unlawful. If it can be demonstrated that LIFO is not discriminatory in age, i.e. there is a healthy mix of ages amongst those with the least service, then I can't see any reason in law why that should not be the primary or even only criterion. I suppose that's unlikely given the traditional progression of cadetship to internal command but it's a thought. Somebody else here may be able to clarify that.