As a developer I am a little bit tired on reading lies regarding the subscription, so let's debunk some myths.
First myth is that the subscription is needed to make the app sustainable. The truth is that app depends on the average revenue per user and its costs. None of this has any direct connection with the subscription model. The truth is that the subscription is most frequently used just as an excuse to hike up the prices enormously as such price hike no one sane would accept in the paid app model.
If we use the example of VDR that has gone from 30$ to 60$/year, that is quite a price hike. Blind people are likely to use it for over 50 years so it costs now at least 3,000$ or at least 100x more.
Second myth is that the money from subscription is invested mostly in the app development. The truth is that literally all indie developer subscription apps have just one business model - paying huge amounts of money for ads to attract users and earn more from subscribed users than what the ads cost.
VDR already buys Apple's ads on the App Store and it is likely that it will invest in other channels like YouTube, Twitter and Instagram.
Getting users this way is expensive - it costs at least 50$ per US user to make an acquisition (for the app that has an expensive subscription, it should be a bit easier to convert users to the single payment app so I assume that their current costs were 15-20$ per user).
Here's the maths behind - Apple charges you 2$ for the US user to see your listing via the ad. You need to pass two conversions to get the user to subscribe - he may not download the app at all and he may not subscribe. For the free app you may get fairly high conversions in the first case (even above 50%), but to convert users into 60$/year subscribers isn't an easy task and 5% conversion rate would be a huge success. And just those numbers bring calculation that costs already more than 50$.
Next myth - the text-to-speech app needs to cost more than 30$ one time payment to be sustainable.
The truth is that this is silly. According to the VDR developer himself and some facts he has spent just a small fraction of his time in recent years in maintaining and updating the iOS app. He said that it took him almost three years to build the Mac app and that in those years he spent most of the time on building that app. While he hadn't posted regarding Android app, it is safe to assume something similar.
On the other hand SensorTower analytics tells that Apple received 50,000$ per month for his app. While Apple takes 15% from that it is still more than 40,000$ that his company received for just a side job that the app was to him at that time.
When the app cost 15$ he gave the interview to one blind blog where he said that his family lives quite well from the app, which doesn't sound like something that has sustainability problems.
I must say that that budget is something that a good US developer needs to earn to be interested in working intensively on the app. However if there is a text-to-speech app from the Indian developer (for example), the app should be quite sustainable at 5,000$ revenues. As such it isn't completely ruled out that someone can make a good text-to-speech app and keep it sustainable at just 3$ price (that would be extremely hard but still I can't completely rule out even that). So again what you paid at that price was not development but rather the US brand and developer's connection to the US community leaders that would promote him day and night.
Clearly the choice is on your side. If you buy the stories and nebulous sentences without any practical proof you may end up financing purchase of developer's Ferrari while you were thinking that he had a hard time to feed the family. Of course this is not just about one app, but about any app out there.