I've created a number of open source projects in my time and it's never as simple as it sounds. In my experience just shoving the code on github is not enough to make a successful open source project. You need to actively put in effort to build a community around it and manage contributors. It's surprising how much work is involved. It really does take more time and resources than keeping the code closed.
I agree that keeping things closed is a bit old skool these days. It really depends on the type of product you're creating though. Some products work better open source and others don't. I wouldn't really call it a disservice to keep the code closed. It either solves your problem or it doesn't. Simple as that.
Maybe, maybe not. Even in the biggest, most successful open source projects the donations tend to be pretty small. Building a big community is definitely a good thing, but usually donations are not the main source of revenue. Often the community is built around an open source free product and the revenue comes from selling other things, sometimes closed source software.
When I look around the icons on my desktop, about half of the software I use regularly is open source and the other half is closed source commercial products. The fact is, I'm happy to pay for software if it solves my problems, even if there's an open source free alternative. Open source is not always the better product.