You start running into problems of parts no longer available and or no one knows or wants to work on a 20 year old car or truck...
Yeah, that parts availability thing *can* be an issue! Especially with my Audis

Some marques aren`t subject to that though; you can get anything for any Benz. As for servicing, that`d have to be taken into account anyhow; in my area there are *NO* Subaru dealerships that do decent work. None. And the nearest competent Audi dealer is well over an hour away (they come pick up the cars). Lots of service places for some things, none for others and I`d figure anybody would be factoring that in no matter what.
IMO most of the time that can be headed off if you get the right vehicle in the first place (e.g., NOT that Mazda MPV I had! And that`s why I wouldn`t own a MOPAR LX-platform) and do what`s called for. Admittedly that`s a *HUGE* "if" though and I suppose most people aren`t gonna do all that.
Heh heh, but to prove your point has plenty of merit- note that I had to have all the brake/fuel lines fabricated for the `93 Audi because the original owner didn`t keep the undercarriage clean enough to prevent rust and the parts are NA.
Safety advances are constant too if you have kids and drive them around rather have them in a decent crash tested modern vehicle ..
I wouldn`t qualify that based on passengers (no one passenger/driver is worth more than anybody else IMO...sorry, that`s a pet peeve of mine; people thinking their "Baby on Board" is more important than somebody else) but *unless* the driver is awfully well-trained and capable that`s certainly a factor if the safety systems are decent (e.g., anybody who can`t outdrive most low-end "skid/stability control" systems shouldn`t be driving period, but I guess they do anyhow). And heh heh, my particular older vehicles are great at withstanding accidents so maybe I tend to discount that. Actually, that`s something that factored in when I was deciding what vehicles to own as keepers, haven`t bought *anything* that wouldn`t sustain a major hit since I was a kid.
Cars and investments aren`t in the same category we could say all that money on detailing supplies each month should be invested too.
Now on that one I definitely disagree

IMO everything money-related is in the same category. I figure that nobody should ever spend a penny on detailing stuff (or other automotive extravagance) unless they already have enough $ that it doesn`t matter. No money should be spent on *ANY* discretionary expenditures until the household`s actual needs are properly funded. I wouldn`t borrow a cent for any vehicle on earth.
Oh, and I`m not being contentious here, hope I don`t come across that way. I find this an intersting topic for conversation as I know people who go every conceivable which-way on "what vehicle and how to fund it". And yes indeed, most of the people I know think I`m a Dave Ramsey-type nutjob when it comes to personal finances!
Eh, some boring, safe lump like my Tahoe or the Crown Vic would be easy-peasey to keep on the road forever with very little cost, but most people want something more special than that. Guess it comes down to "how special can ya afford"?