It was an introductory move. I do not have any issues with it.
Thanos was spot-checked for a decade before getting the spotlight. He finally grabbed center stage in
Avengers: Infinity War after a brief post-credits cameo in
The Avengers. Even so, Loki was made numero uno.
My complaint is that Zack Snyder knew what he wanted to do but Warner Bros disagreed.
First, they swap Snyder out for Joss Whedon, who went with the villain you disagreed with but offered NO follow up for future projects. Second, the studio backtracked and re-installed Snyder, who cleaned up the villain you do not like and established a path beyond the movie. Then Warner Bros squashed everything and was left with egg all over their faces,
that they threw at themselves.
It is like the real multiverse of madness exists insides the skulls of that studio's executives.