The amount of people who's games would be improved by 200 monsters but not at all by 100 seems exceedingly small, and as already mentioned there are hundreds of professionally crafted monsters produced for 5e, they just spent an episode of their official podcast on ToBs.
As to the second part, that is a strange way to look at it. Nothing is forcing anyone to re-write all the fluff if they don't play FR. If it is useful to someone but doesn't fit their setting then it might behoove them to makes some changes to the tables for instance, but why in the world would they rewrite fluff they already have ie; if the fluff on giants doesn't match their setting in a way that is significant it would be pointless to rewrite it, they will just not use it all. On the other hand, for many GM's working in a homebrew setting they can simply use this stuff when and where they want to. Most settings aren't defined by the fluff of every particular monster. The majority of these monsters can be plopped into other "official" settings, all the fluff intact, and work great.