Cap'n Kobold
Hero
Because they will increase general party power to a large extent, and also shift the balance of damage capability severely towards casters and other long-rest based classes.Yea. How do those assumptions really change who the best damage dealer is?
More combat rounds gives the classes with less nova capabilities, but more at-will damage a chance to catch up: Initially the nova classes will open up a large lead by throwing high-level spells, burning sorcery points etc. Once they are down to cantrips or similar, the less-nova classes will start to catch up. If the adventuring day is so short that the nova classes aren't forced into using their lesser, at-will attacks then the other classes aren't going to be able to catch up, and thus will be much lower damage-dealers.
Similarly with removing short rests. If a DM runs no short rests without adjusting anything else then classes that recover resources on short rests are going to be operating with less than half of the resources that they are balanced around. Thus the long-rest-based classes are going to appear to be much better damage-dealers than they would be with the original set of assumptions.