While it's always possible to keep an old vehicle running, with a large enough influx of cash and time, it's generally not productive.
Vehicles worthwhile doing this for include old muscle cars, other old classics, and many sports cars. Vehicles which are simply consumer commodities are absolutely not worth the effort. In fact, it makes a whole industry rich, supporting the old cars as people throw money at old clunkers.
I see it all the time, $3000 worth of repairs and then it'll be good... the thinking goes. And next month, it's something else. Within 6 months, it's gone. The entire time, it's provided poor fuel economy, the AC does not work, handles like a boat, the drivers seat is uncomfortable and so on.