It won't work because people always want the quickest/most tome options. Add in two new dungeons, those dungeons tend to offer best tome acquisition on top of Expert.

75 from doing daily, but then say the new dungeons had +15 more weekly tomes you could obtain. Then you could get it done in 5 dailies rather than the current 6. Slight yet huge change that would be lost if the current dungeons were part of the expert roulette because then it would just be a game of luck if you would get the 5 runs to cap or have to do 6, because 1 roulette in which you did the 2 current dungeons would mean there is no point in doing the higher dungeons because it'll take 6 runs to cap regardless.