So the biggest strike against Kytheon, as a commander, is that he isn't a win condition, nor does he tutor or enable a win condition. The importance of this cannot be understated. Making your creatures indestructible is, typically, not very useful in CEDH. Additionally, If I were going to be making a mono-white CEDH deck, I would want my commander to, in some way, slow down or prevent what my opponents are trying to do. In this regard, Thalia or another hatebear would be much better in this role.
Another strike against a deck like this is that it has to be mono-white. While mono white has some very good CEDH staples, they are often a very small percentage of the overall makeup of a deck. White offers essentially no card draw, no counterplay and very little ability to tutor or ramp. This will drastically slow your deck down.
Remember, you are going to be fighting against decks that are doing very powerful things in the first few turns, and many can win on turn three or four rather consistently. You can't hope to race that type of speed and deal 120 combat damage before one of your opponents win.