I want to expand on this. Not only do I agree with what you wrote, but this app is trading access to millions of available sources in a very well known writing system to one more or less unique to this app. That's a terrible investment for anyone who wants to do more than learn 1-2 favored songs.

If you need to sight read (and as rock/pop/jazz people point out, you don't have to for many genres), then you need to sight read.

There are so many other virtues to sheet music. Look at the cover image. I can see a few notes. I can see vastly more notes in sheet music. I can easily evaluate if the piece is playable, I can scan and look for broader patterns. I can see that a bass note is being held for 8 measures (and I may choose to repeat it at some point). I can look ahead quite a bit. I can understand the repeat structures - don't gasp, but you don't have to take repeats, or you can repeat more times than written, especially with 20th+ century music, where you are often expected to do things like choose your own ordering of measures or blocks of measures. There are fingerings. I can see if the composer is writing out finger pedaling explicitly (Couperin normally does, Bach normally doesn't). I can see the pedal markings, general contours of dynamics. I can see the trills, etc., which are often just suggestions rather than hard requirements. I can see the meter, meter changes, keys, key changes, accidentals. I can see a big scary chord coming up and spend a bit more time looking at it while I play a few measures behind. I can see that Bach is repeating a phrase a 5th down, or inverting it, or reversing it. I can see the difference between passages meant to be played in time, and fioritura type writing.

I haven't used these piano roll systems so there are undoubtedly some things that are nice about it for an experienced player that I don't know about, so that paragraph is one sided. But that one side is very important - I'd loathe to go without them, and can't imagine I'd ever trade them for whatever advantage the piano roll might bring. After all, a player can take a sheet of paper Chopin wrote, produce that music at a more or less performance level. So it gives you about everything you need. I could imagine a current composer might find something more expressive about the piano roll (maybe expressing note durations not evenly subdivided by 2 or 3).

I suspect there is something neurological happening that stops some people from sight reading well, just like some people struggle with text. I've read accounts of people trying for years, with seemingly good practice techniques, still struggling.

So things like this, synthasia, etc., seem to have a niche. But in general, I suggest, think about someone proposing an app that instead of displaying printed text output it sonically. Great boon for certain situations or people! Undoubtedly someone is using one to read this very post. But a terrible replacement for reading in general.

If a six year old was relying on screen readers because reading is too hard to learn, after testing for dyslexia and vision problems, you'd urge them to make the effort; the advantages of reading text vastly outweighs the 1st grade difficulties of learning to read (yes, that time span will differ by language and writing system, not the point). Literacy is empowering, and arguing that the auto mechanic down the street can't read yet makes a good living is probably not a convincing argument to not teach a child to read.

I learned to sight read at age 4-5 with a plastic brain (I recall my mother having to teach me the letters a-g, and how to write them, for example), so I may underestimate the difficulties of learning later in life. But if you are in a situation where some kind of notation is helpful (again, not all are), learn standard notation!

edit: I thought of a counter-example. Say you play in a band. You can record your output to midi, and then share it with others. You can quantize midi and turn it into sheet music, but chances are you playing is not rhythmically exact. Sight reading that sort of thing is painful (notes carry 1/16th note into the next measure, that sort of thing), and I imagine a piano roll would often be easier.