There is no evidence that long term use of nasal sprays cause significant systemic side effects. Some people do take Flonase for the rest of their lives...

The most mild reason is you may be wasting money on something no longer required.

A concern is it can be masking more serious underlying conditions or the reason you are using is has gotten worse, which should be evaluated.

For instance, Flonase can relieve some issues caused by nasal polyps (little bits of skin growing where they shouldn't be). A better long term option may be surgically removing those. Your family doctor will inspect and compare their number and size to previous observations.

It's always good to check if the reason using are using those long term medications has changed.