Yes and no. I would upgrade your components as they wear out or break. For example, I picked up a stick and it got wedged in between the jockey pulleys of my ESP 9.0 rear derailleur. It literally ripped the derailleur off my bike. To replace it, I bought a newer 9.0 SL for only a few dollars more. Same scenario when I tacoed my stock Sun DS-1 rim. I upgraded to a Mavic D521 rim.
To do a wholesale upgrade of components or switching to discs, you are correct. It would be more economical to sell your bike and buy new!
L8R
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger