People probably have an issue with it being the only thing being added to a season. “Do the same things all over again, but with DIFFERENT powers!!!”
They also need to make every seasons seasonal powers better than previous. If they are less fun than a previous season some people will not bother with the season. They do need to keep that in mind. “Good enough” will never be adequate. Strive for greatness, Blizzard.
The more players you maintain the more your next expansion will sell. Some people will quit and not care how enticing your next expansion looks. They will have moved on. You having lost their faith.
Make the game great every chance you get. Don’t have a “good enough” attitude with your seasons.