Use a server or not?

I kinda want to use a server for my Windows Universal app, I'm just learning C#. So this will take awhile but getting better at it. I don't the app to make tons of money, but i think I will get sales and it will be good practice and something to show future employers.

 

I also want to make a web app after the phone app so I can reach more users. But how does it work, say a year down the road and I don't get sales anymore, but I'm still paying for the server? Can I take the app down even though the users paid for the app? How do apps keep keep going for years? Or do they get enough consistent sales to keep going, thinking if they paid for the app I can't just take it down...

 

Going to keep going and not worry about the details so much, but that crossed my mind.