The one thing with ZenCart is it not only has its core team developing things, it has a whole community that makes modules and advancements to the software. What Zen Cart does then is release their new version after they make sure everything is secure and in working order, or they make the modules available on their site after they`ve tested to make sure they don`t break the system and don`t compromise security.
So in essence, much like many open source projects with good communities, you`ll always have people helping make the product better. Also, even if they took down the Zen Cart site tomorrow, the functionality of the current installed software would not change. Not to mention with all the active programmers on their boards, I`m sure they`d just find or make a new site where they can continue advancing the system.
I`m glad you did bring up that point though, because it depends on how any cart selected works what will happen if the company that produces it closes shop. Example, is you have a cart that is not open source and you pay x amount of dollars for every year (there are some that do this) and then that company closes shop, come the end of your term, if the system is programmed to not operate without getting a new license code or connecting to their site to confirm payment, you could wind up out of luck. The same results can happen is you get something like prestocart or volusion, where your whole cart system is hosted on their servers, if they go out of business, you could just loose everything.
That`s why I`ve generally suggest open source, due to the ease of modifying it to fit your needs (both on the backend and frontend), the ability to add custom modules created specifically for your site, and the knowledge that unless your hosting company closes shop you shouldn`t have any problems. In fact, I just did a zen cart move from one server to a new one the other day because my client decided he wanted a new hosting company. The transfer involved me copying all the files from the server, making a database backup, uploading to the new server and changing 3 or 4 files and it was active and working again. The hardest part of the would have been the database backup, but since most servers have access to PHPMyAdmin or similar Database GUIs, its just clicking a few buttons to get the files, and clicking a few to put them up elsewhere.
It`s that ease of setting up zen cart (which can be done most times through fantastico if the server has it), of developing and maintaining the system, and just known customer experience of the cart has always made it one of my favorite carts.