"it's very efficient, nobody would ever think to steal or break into it as long as there's pretty much any other car in sight"

My impression is that the most stolen cars usually fit that description, surprisingly enough. Old, reliable, nondescript cars are exactly the ones that have a thriving market for parts and are relatively easy to steal. I remember for instance, late 90s Honda Accords being on the list of top most stolen models at one time.

The other reason I see not to keep an old car forever is that I don't trust old airbags, especially since the Takata fiasco. Nothing lasts forever, and originally I read that airbags were supposed to last 10 years, so how do you trust a 15 year old explosive device aimed at your face?