That would be totally fair, since the old players have already harvested the MA prices for their sets all those years
That is not fair at all. The assumption is that everybody regardless of how long you been playing and how much you invested and how many hours you play, has the same rights (on buildings) within the game. Which means that if I would check in every other day for 15 minutes I would have a 'right' to gain the same rewards as someone who checks in daily, plays gold in the spire and is permanently in the top of ones world in tournayscores.
So what if you have been playing several years and you have more stuff then newbies. If you spend a lot of time trying to learn as much as you can about the game, the spire and the tournays are easier then for newbies.
The whole issue with the moonstonelibrary is that the devs did not think about it enough to avoid the obvious mistake: overflooding the market with scrolls.
You cannot correct that by taking something away which has been won or given. But a change like the one that has been made will have a possitive effect on the market in the long run and that is what it is all about.