Having calmed down and read the short article explaining why, I can understand the decision.
When I started playing league of legends years ago, Dominion was definitely a positive experience for me and helped me to solidify my decision to keep playing this game. Would I have abandoned league of legends like so many other games over the years if it hadn't contained the Dominion component?
Also, my wife hates summoner's rift for some reason I can't understand, but likes ARAM and dominion. I'm sure there are some personalities to which summoner's rift doesn't appeal. What is 0.5% of 32 million players? 160,000 players. I hope rotating game modes every 3-4 months are enough to keep that type of person interested in league of legends.
RIP dominion.