I think you are also really down playing the convenience vs. traditional car rentals. Many car rental companies have moved to the model where they offer you a certain class of vehicle, and when you get there (which may mean traveling for 30-60 minutes to a place like the airport) to pick it up you are choosing from a random smattering of vehicles, which may be close to what you are expecting or not. I certainly don't appreciate when I get an "upgrade" to a giant SUV because they were out of the compact sedan that I really wanted. With Zipcar, if you live in a city, you can find a car in your neighborhood and know exactly what car you are getting, down to the color. You can also choose vehicles with other upgrades like roof rack or state parks pass. Zipcar also makes it easy to have multiple drivers use the car. Let's say I want to let my wife or brother drive during part of our trip. No problem, they are both Zipcar members as well, so they can just drive the car. They don't have to be there when I pick up the car to show their license and get added as drivers on that specific rental.
For me personally there is also a huge benefit to Zipcar offering cargo vans as a rental option. I'm a hobbyist woodworking and having quick access to a van to pickup lumber is great! None of the other Zipcar like services that are available to me offer this. I could of course get this from a traditional rental car company, but with the problems from the previous paragraph.
I think my point is really that Zipcar hits a sweet spot that no competitors seem to be addressing directly.
I don’t want to wait in line or deal with their representative upselling me a thousand addons, I don’t want to think. I’d rather show up and hop into a car, a-la ZipCar, without having to spend 20 minutes talking to anybody or figuring out what insurance I should choose.
I do have to believe that the (subsidized) Uber and Lyft rides has had an impact on their business.
And this is a business that probably didn't end up being as big or disruptive as people thought it would be in the first place anyway.
???? Zipcar has allowed you to rent a car for more than one day for years. I know people using them for 3 day weekend trips a decade ago. Explicitly remember them trying to budget their miles as you only got 180 miles per day, which you can burn up on distant destinations.