There are also YouTube videos that simulate the app's display experience without the training and feedback of course. Here is an example. You can search YouTube and find more examples as well as instructional videos for playing with just the YouTube videos.
The forerunner of Synthesia, Piano Hero, was only recently invented (by my standards) in 2006. Whereas the early incarnation of it was free, the renamed Synthesia app was popular enough that it is now being sold. How well is unknown to me. But the point is that this app demonstrates just how quickly clever and fun ideas like this can catch fire on the internet.
Of course, Synthesia is not really sheet music notation, but it is a tool that supports learning to play music in a fun and intuitive fashion. (Even the URL itself suggests it’s just a game: https://synthesiagame.com/). And interestingly, a Klavarskribo Facebook group has named itself "Synthesia on paper!". That pretty much illustrates their similarity, it's just that one is static, the other dynamic.
Today on YouTube, there are lots of pieces of music as well as instruction on learning to play the piano with Synthesia. The YouTube view counts are often in the range of tens of thousands to millions. (Admittedly, I'm guessing that it’s likely that a lot of these views are a result of people who are just mesmerized by the falling bombs and aren’t using them to play the keyboard.) However, this demonstrates that some find it an easier and more enjoyable way to play music the music they love. And my guess is that these people are young people who are accustomed to using apps to play video games on display devices (PCs, iPads, smart phones, etc.).
But does Synthesia replace the need for an easy-to-read Alternative Notation? I don’t think so because it is tied too closely to the app and/or the YouTube videos. Thus, it does not provide a general solution for all music and all instruments. And if one uses it just to memorize individual tunes from YouTube videos, it really does remove any connection to music theory. Having said that, there are surely those who are happy with this capability to learn to play a tune easily. However, others might prefer to try an alternative notation if it were really easy to read.
The most important lessons taught by Synthesia are:
1. It demonstrates that there is an interest in an easier way to learn to play music.
2. It shows how ideas can be quickly spread on the internet and be widely adopted thanks to websites and social media.
3. it proves young people will experiment with new tools, techniques, and apps.
4. it proves young people are not bound by tradition.
And thus, the take-away is that a well-designed and easy-to-read music notation app might also appeal to a large segment of Synthesia’s users as well as other beginners. Especially, if it also serves as a tablature. And if properly developed and promoted, the app and its notation could also spread like a meme on the internet.