I'm sure they will, too.
60 songs can easily give them a few more albums at least, maybe four, plus remasters or whatever else they decide to release.
Even 40 songs would be quite fine. Most artists who pass away wouldn't have that many songs just laying around. I think maybe you heard a rumor?
Michael worked on 'hundreds of songs', yes, working on his albums, but not that many of them were nearly complete or ready for an album. In fact, many of them were unusable, of all the hundreds of songs being worked on, only a core 30-50 would be narrowed down and probably really worked on, then he would have to write complete lyrics to them and the ones that actually ended up getting complete lyrics would likely be more around the fifteen to maaaybe twenty song range, I don't think Michael was notorious for finishing lyrics to songs as we can see in many of the demos that are out. He more liked to feel the songs and just sing the sounds he heard in his head rather than the words, which is fine, but they aren't really releasable in an album format.
Many people close to Michael or who worked on his latest album, work for Sony, whatever, have stated there's only about 40-60 songs left that can be used. I believe maybe it was an author of a book who stated that recently or it may have been Tommy Mottola, I think maybe it was an author though. They stated that really there's about 60 songs left to be used and not all of them can even really be used because of the whole incomplete lyrics issue...
That makes a lot of sense to me. I can see Michael working on hundreds of demos, yes, I believe that, but not that many that can be released... 60 demos worthy enough of release on an actual official Michael Jackson product is quite a lot, believe me, they can drag it out.
700 songs just seems so unrealistic to me in terms of actually being complete and ready to go, but I await your source.