It depends on what caused the discoloration. If you have a skylight it could be UV damage, in which case cleaning it might not help, though some people swear by peroxide in these cases (I’ve never tried it).
If its mineral deposits from hard water, then Bar Keepers Friend and some elbow grease should get it off.
I painted my shower white again using Rustoleum marine paint. Its held up nicely for years now. Was amuch cheaper option at the time thna replacing the entire shower, but takes some prep work before you apply the paint.
We replaced our shower lining last year. FWIW, I remember reading in the manual to clean it with a vinegar and water mixture to remove mineral stains. Might be worth a shot before replacing/painting…