The OP's tent had a lot of use compared to normal.
I have some new Hilleberg tents, which are silicone treated on both sides, but I fully expect them to break down with UV light. Polyurethane treatment will be less effective, cheaper and still suffer from UV light.
I have bought some tarps to protect the tent when weight /space is not at a premium. The tarp will also keep the tent much cooler inside during the summer time and probably quieter during rain.
I do have a very old Vango Stormforce 10 MkIV tent, which is cotton and I would think that could be successfully waterproofed, but the material is heavy (technical cotton) and also not terribly cheap. But in any event, I would expect that the cotton will become less structurally strong with age under UV exposure, so has a finite life. As an aside, if you ever looked at the lint filter in a tumble drier after a drying cycle, you will see a load of fibres removed from the clothes. I would guess a tent would similarly loose fibres in use.
So, I personally, would plan to replace the tent after a lot of use. I have been told by my Outdoor Shop that it is not possible to successfully reproof a silicone or polyurethane treated nylon tent, even expensive ones. They also warned me to be very careful if cooking inside the tent as the silicone treatment is very flammable.
As said, boots, jackets and other outdoor gear don't last forever.