It's not just a problem of stuff getting acquired. Companies exist to make money, and they have to chase new customers. No matter how good the software was originally, sooner or later you're going to stop being the target demographic. This happens all the time, every single proprietary product I've used eventually changed in a way I did not want it to change. At that point I either have to adapt or find a new product. On top of that, companies go out of business all the time. At that point you lose support for your product, or if it's an online service the product itself disappears.

With open source the situation is much better. As long as there's a community of users who want it to work a particular way, then it's always possible to fork it and keep it working the way you want. A perfect example of this was when GNOME started moving in a direction a lot of people didn't like, and now we have Cinnamon and Mate desktops.