It's a pretty huge nerf for advanced 10 chest players.
E.G. 52 BP per year and 2 guest races per year make it pretty easy to spend all BP on magic buildings for spenders. Cutting the value of a BP in half means an additional upgrade cost of 8,320 diamonds.
I saw a post on beta (I think it was edeba). Its definitely a nerf to players who had all or a majority of magic buildings, AND only upgraded them via tournaments. I think that's a pretty small subset of players, though.
But like I said, its a nerf numerically, but its a buff otherwise. For players like me and you, who have 1-2 magic buildings, but maybe have 20+ blueprints, you went from having 2 easily upgraded buildings and a bunch of blueprints collecting dust, to having 2 moderately more expensive upgrades to your buildings, and now a lot of other buildings that can also be improved.
Now, given the event building nerf from last winter/spring (God that felt like forever ago), maybe the conversion number is too low. I think there's a good argument for that. But I think my point is that there has to be SOME discount since now BPs are incredibly more versatile, and so the question is whether a 45% nerf or some lower number is more appropriate. But that's all just personal preference, and of course players are going to argue for a lower number (and Inno will want a higher one...).
Another point to keep in mind is that I think it was only a matter of time before blueprints got their value reduced. 10 chests in Dec 2018 is very different than 10 chests in summer 2016, right? Between wonders, buff buildings, more analysis of fighting/catering. If anything, maybe this means that we get an 11th chest that has 9 more RR spells, or something like that.