Be careful what you wish for.
The up-or-out system isn't perfect, but it creates problems for promotion opportunity. You think it's hard to make O-4 now? Just wait until your only chance is to wait for someone to retire, and even then you've got to be in the top 1% of fitreps to make the cut. Promotion opportunity for officers was an issue in the early days of the country, and the system we have in place now was put there to fix it.
Better yet, wait until the only chance you have at even getting a commission is to be put onto a huge wait list because there are too many LTs who won't retire.
This was an issue in the early days of service, and can also be an issue in other areas of civil service like police officers and firemen. Yea, there are some people who don't want to be promoted to a 'desk job,' but there are many more people who would enjoy the opportunity for advancement (or even to be hired for the job) who can't.