Nothing lasts forever. Repelin needs to be re-applied every year, twice in the tropics.
Poly Guard protects the tubes from UV, oils, dirt, grime, etc. It is also required as a primer or base coat for the Repelin on the tubes. If you put the Repelin directly on the tubes, it will cross link to the fabric and will be difficult to remove if ever you need to do a repair or patch. By putting it over the Poly Guard on the tube, it can be completely stripped if necessary.
All surfaces need to be cleaned and stripped with Speed Clean prior to application of Poly Guard and Repelin, even brand new boats. This insures that the surface is free of all dirt, oils, release agents, oxidized material and other pollutants that could impair the application and bonding of Poly Guard. When you clean your boat prior to the next application, you will strip the Repelin and Poly Guard so that new material can be applied to your boat and not to the oxidized and spent protectors.
Layering new polishes or protectors over old is false economy. The new is protecting the old, not the boat. It will break down faster and delaminate from the boat, leaving it completely unprotected. Think of using expensive paint to paint over dirty, greasy, peeling wall paper on a wall. You're better off to strip the old paper and applying a primer before painting the wall. The wall will look better, be better protected and the paint job will last much longer.