Keep/upgrade only if you really love your old bike.
Here's a copy of a post about mine (Parkpre):
My hardtail ride is the same bike that I bought in 1994- Parkpre Team 925, steel frame. I've taken good care of it and put many thousands of miles on it (less miles nowadays 'cause I also have a FS). There is absolutely no component on it that is original, with most stuff having been replaced multiple times. I'm actually on my second crankset (rings replaced many times), but only because there were grooves from my shoes wearing the sides of the crank arms. I love my steel Parkpre.
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"I tell you, We are here on earth to fart around, and don't let anybody tell you any different." - Kurt Vonnegut jr.