I get asked that by desktop software people a lot. While the one-time fee wasn't my most brilliant business model move ever, there is no structural problem here. Very, very few of my customers use BCC on a year to year basis. (I have a loyal few dozen, but something like half use it twice or less.) If it became impossible for me to sustain BCC going forward, I'd turn off sales and then gracefully shutdown after six months of notice. If they complain after that, I'll write them a check for their purchase price so that they can use one of my competitors.

However, since software doesn't rot and the BCC monthly server fees are mouse droppings even when it is going gangbusters, I could probably arrange for someone to support it for my entire natural life and beyond.