The facts speak for themselves. He stayed with her for decades and got her pregnant multiple times (she clearly didn't withheld sex then). No one forced him. All the other marriages combined are not enough in duration to even compare.
Also, he could have seen the light and been in the tunnel if this was just about death. Showing Stephanie was more than that. It signified that no matter what, she will always be The One. If Donna or Brooke were his true loves/favorite wives, he could have heard them urging him to fight. Lots of fun with the honey jar waiting for him on the other side of the tunnel. He'd come back for that, trust me.
It's unfair to judge a marriage by its last years (when Stephanie was much older and had already given birth to five kids), compared to young, opportunistic women who wanted his money and status most of all and were willing to fake it till they make it. And also... Eric thrived with a controlling woman. He needed a strong female to fix his problems. The others were there to boost his ego and a healthy ejaculation. Just think about it - while with Stephanie, he built a dynasty... with the others - it crumbled. I can't forget that he lost the company right after his time with Brooke.