Not so. If you go through IMSLP, a surprising amount has been contributed in the form of digitally typeset files in Finale, Sibelius, Lilypond or MuseScore formats, usually with the typesetting files available alongside the PDFs, especially for popular composers like Bach. (That was standard for the Werner Icking archive, and IMSLP took over that archive whole.)
Now, if you were proposing to do it for underexposed composers, maybe something like the complete Strasbourg MS of J. J. Froberger or the collected piano works of Nikolai Roslavetz (who is PD here in Canada)...
Bach doesn't need the advocacy, and focussing on composers who aren't so exposed might actually make clear that there is something for everyone's tastes, not restricted to include just the canonical Great Men. (In many cases, fame isn't just about talent or genius anyway: it includes exposure, and a person like Roslavetz, as an example, being on the wrong side of Stalin, just wouldn't get that exposure.)
Doesn't much matter, really. I still use the programme (an older version, no less) - it does what I need it to do, and it does support a MusicXML plug-in if I need to exchange a score between programmes. If I need to display the output publicly, I create a PDF, which is as bog-standard a format as can be had (even though it is proprietary). When the available operating systems no longer support it, well, I have older hardware stashed away - it doesn't "chug" on older systems.
In many regards, it doesn't really matter whether the software is open or not, provided it supports standards (open or not) that really are standard. One of the points where these "playable scores" fall down, however, is not in presentation of the score, but in that the standard for playback, quite bluntly, sucks - General MIDI makes for a bad (and entirely too variable) representation of the audio. I suspect that is why Score Exchange has gone for PDF + MP3 - it isn't that much work to flip the pages while listening, but the audio is likely to be more representative. (Edit: One of the reasons, I should say - the lack of support on Avid's part for the Scorch plug-in was the main reason, true enough, but many of the user community had been pressing long before for something similar anyway.)
I have my sincerest doubts that following along with a score will ever be popular, whether you have a large library or not, easily presented in a browser or not (and achieving that goal requires rather more than exposing works that are extremely well-exposed to start with - see above). It would require cultivating a culture of literate music-making, the production of written music (whether as performer or composer, or both), in a society that is awash in music consumption. That sort of relationship to the score was lost when the parlour piano went out of style and these newfangled devices like radios and phonographs took over.
I'm actually agnostic about the relative merits of written music vs improvised music or that in an oral tradition. Good music can be had either way. I just happen to fall into the literate camp because that's the kind I find in me to do. There will always be people like me writing music, and always at least some audience for us. However, it does take considerable effort to learn the ropes. In a parlour piano culture, that effort is just part of daily life (i.e., if you don't entertain yourself, who will?), but in our society, not that many people (relatively speaking - still quite a large number in absolute terms) are willing to spend the time needed, and they have ample means to consume music without that effort. In popular culture, they have ample means to make music without that effort as well.