It is a shame that so many good app designers/developers are leaving the business or going to the subscription base service. I am not a fan of subscription base service and told myself I would never use them for apps. I have evolved on the issue, but the app needs to be extremely useful. I currently subscribe to two Officesuite and Report Plus. I would subscribe to your product as well if it was offered.
There are some app developers that offers both subscription based service and one-time pay. I will describe the app Moneywiz because I most familiar with the apps pay options. MoneyWiz offer a weekly and yearly (cheaper) subscription options. With either of the subscription option, subscribers receive minor and major updates for free; the app includes iPhone, iPad, Windows and Mac versions; and special banking features not included in the one-time purchase. The one-time purchase includes iPhone and iPad apps only. Windows and Mac versions are an additional purchase each. One-time includes all minor updates for free and a discount on major updates, which are schedule every year and previous version is supported for 6 month after major updates. One-time purchase users are notified, when a major update is coming out and the sale period, which is extremely short.
Now for some constructive criticisms.
I have been using your app and window base software for over 5 years. When I first saw your app in the app store, I went to your website. The site looked out-of-date. If it was not for your forum and the response after a left a message, I would have move on to another app. I image many have. The second criticism, which is also a compliment, is your app has not had a major update (features and appearance) since I purchased it. That said, your app is still the most function rich universal database in the app store. However, the graphics look mid 90's. There are a lot of flashy database apps in the apps store that have one or two nice features (iCloud sync, over the air sync, barcode data input from the internet, updated visualization charts and graphs, etc...). I hypothesize that if the app graphics are updated and the price was cut by a third sale may increase.
I hope you are able to continue to support this app and makes some money. I believe there are many loyal users who will continue to support your app if you transition to another buying model.
good luck,

Jay