First of all, the days of getting a decent car for $5k in the US ended with the pandemic and the supply chain insanity. But even if you could do that, you also need to factor in:
* Gas
* Insurance
* Registration and taxes
* Maintenance
If you don't use a car as part of your commute or regular errands, it's likely cheaper to rent.
Honestly, the best compromise solution is usually to share a single car for a household. Not helpful if you're single, but if you have a partner, then you can usually get by fairly well with just one car and now your individual costs are cut in half.
Another way to balance the trade-offs is to own a car that's smaller and more cost efficient but doesn't cover all your needs. Then rent on the infrequent times when you're traveling, camping, bringing a bunch of kids to a party, hauling lumber, etc. and you need the extra storage or passenger space.
Regarding the sub-$5k car: it depends on your definition of "decent used car." Most people are unwilling to accept a used vehicle with a slightly broken interior or other minor issues. Just today, I was looking for comparative prices for a 2003 F-150 that I've got, and I found multiple in the $2-3k range in my area that are in good condition (running motor, A/C works, non-necessary repairs required). And that's a pickup truck: if you want a coupe or sedan, there are even more options below $3,000. You just need to be willing to accept some flaws: right now I've got several tabs open of good "A-to-B" cars with under 200k miles.
And at $250ish to rent a vehicle for the weekend it only takes a couple weekends to pay for the insurance, registration, and taxes (or is it just that much more expensive on the coasts? I'm in the midwest). There's a reason people own instead of rent, and it isn't because renting never occurred to them.
The only difference is that Zipcar has a much stronger market position than an individual consumer, and thus can get better contracts & economies of scale for things like purchasing, financing, maintenance and insurance.
And there's no reason to think profits are a very significant part of the pricing. It's a competitive market and Zipcar has never turned a profit in the past 20 years.
There's overhead both for Zipcar and for consumers, e.g. to arrange for repairs on cars. Consumers are just doing 'unpaid labour' by arranging a car to get repairs for example and pay in 'opportunity costs', while Zipcar will pay staff to arrange such repairs, and charge the customer ultimately to pay their staff. But economically there's no difference, opportunity costs are still costs. And again, I'd expect Zipcar to get economies of scale, dedicated professionals working with optimised processes and tightly negotiated contracts on the overhead, to do better than an average joe having to arrange the same repairs.
So while I won't argue the $240 bill you referenced, or calculate exactly the equivalent costs of a personal car, I hope you'd agree there's no fundamental reason equivalent use of a car via a sharing service should be more expensive.
I do hope you agree that it should be as cheap, or cheaper for two reasons. One the economies of scale & professional approach to car ownership, being more efficient. But two, that some of the costs (like manufacturing the car) don't scale with usage, and thus become way cheaper when spread over multiple users. If you only use a car 1 out of 10 days, there's no way I can see how a personal car can get close to as cheap as a shared car.
So apart from personal convenience & psychology around personal ownership, I think car sharing wins as an economic model long-term. Just like we share airplanes, hotels etc. Now convenience wins because there's not a car available everywhere and always. But as soon as every car has self-driving tech, given the amount of cars I see around me in a city that are just sitting there, I think in urban centres car sharing will become the norm. In less dense places personal cars will likely remain.
I don't think people arguing this have ever actually run the numbers and realize just how expensive rentals get. Every year my wife and I plan a week long road trip. If I price out this years trip from the midwest to Utah in a full size suv or truck (need 4wd and space for climbing/camping gear):
Base rental: $970
mileage fees: $350 (1400 miles at 25 cents a mile)
Tow fees: $180 (optional, but I like to bring my small teardrop so $20 a day for the tow package fees)
We're now close to $1500. At this price I can buy a truck, title it, insure it, and then sell it at a slight loss when I get home and still come out ahead of the rental. In just one trip.
*edit - formatting
I've lived car free in multiple us cities and the whole rent a car for a road trip thing was always a huge pain.
Want to pack the car the night before the trip so you can leave right after work on Friday? That'll cost extra.
There are just too many little pain points to make this something we would do very often.
I feel like this is the missing piece for me for the car-less life I dream of: what to do when I want to take a few-day trip to any of the beautiful places in easy driving distance of my dense urban core?
Car sharing is an option, but it's quite expensive. If I take a zipcar for 1 drive out to a campground on a Friday, then 1 drive back on a Sunday, I'm spending $240 or more.
You can argue that the cost of car rental is way less than the amortized cost of an actual car, but a $5k used vehicle pays for itself after 20 such trips.
I don't know a great solution here. Trains + long-distance buses only work if you're going exactly where they're going, or if you're headed to another city with good post-train transit options. Rent other people's unused cars (à la Getaround)? Prices for that seem to be barely better than Zipcar. Maybe rentable driverless cars, eventually: they can drive me to the campground, then drive themselves to the nearest town and rent themselves out to other people for the weekend until I need them again? I dunno.