And when you step back consider other things. If your car is immaculate in every other respect then maybe the cost of rust treatment or even welding is justified But with a car thats at least 14 years old there could be other large costs on the horizon. New tyres, battery, exhaust, further rust, lots of other mechanical problems it may be better to cut your losses while still ahead .
Rust treatment may mean it will pass several more MOT's . If you are lucky it may mean it passes the next MOT with no rust advisory. This would restore the value of the car and make it more saleable. If you decide to sell once the job is done it may be worth getting an new mot done early
With a rust advisory potential buyers will value it at not much more than its scrap value. Some may hope to do it up themselves and get a bargain, but they will still expect it cheap.
You might only get one year, but potentially several, until it fails its mot, on this or other issues and you again need to reassess whether its still worth repairing. The car will be that much older, and the decision easier to make, but you dont want to throw good money after bad on costs that are ultimately wasted.