I have actually done that. Unfortunately one bad bearing means the rest of them are going to follow. So you could get by for a while but it's hard to say how long. You don't have to replace the syncros as long as it's been shifting ok. Replacing the syncros is the biggest pain in the whole process. You can certainly replace just the input shaft bearing if you want, there's just no telling how long the rest of it will last. However if you replace the following while you're in there it will probably last until you get rid of the car. I suggest you open it up and inspect the following. If they feel loose just replace them while you're in there. If you're lucky it's just the input shaft bearing. But chances are the other shaft bearings are worn out (the diff bearings might be ok).

counter shaft bearing

input shaft bearing

main shaft bearing

counter shaft bearing

diff bearings