What you're describing is a super elite defenseman, and I don't think a team inherently has to have one to contend. For example, Florida had a strong committee last year, and Carolina won a championship without one years ago. You did mention Ekblad with high regard, but I don't see him in that category. He's clearly a top pairing defenseman, but he was part of a strong pack playing alongside Forsling (who played the most minutes in the playoffs), Montour (top powerplay unit quarterback), and OEL. If the Blues can get a top pairing RHD to play alongside Broberg, while Parayko and Lindstein handle the second pairing, then I think that would work assuming the forward prospects are developing as expected. Maybe Army/Steen can pry Simon Nemec out of NJ for a nice package involving Bolduc and another forward prospect(s).
As a side note: Slavin and Parayko have virtually identical production over their careers so far...
Slavin 282 points in 701 games (0.402 ppg)
Parayko 278 points in 696 games (0.399 ppg)
That kinda means Parayko is a bit underrated league-wide since he's already in the upper echelon defensively.