I do think they were looking for the rental market, because the best options were rentals and it wouldn't cost you Bo/Girard. Now, things have shifted. Other than Henrique, there are no good rentals out there. So need to dig deeper. And it would require trading out Bo/Girard as the main piece. There is no reason to go after Tanev other than the fact that they are moving out Bo/Girard. I don't think getting another D-man is a bad idea, but of Tanevs caliber? Nah, absolutely no need.
I dunno, Tanev makes sense if they want to get A) A heavy PK minutes guy, B) Move Manson down to 3rd pairing, C) Another right shot.
I think a Tanev add made a lot of sense even with no intentions of trading Bo/G. Tanev would have perfectly balanced the Defensive roster heading into the playoffs and allowed 3 Top 4 quality pairings.
Toews - Makar
Girard - Tanev
Byram - Manson
With a LD and a RD on each pairing, a strong puck mover on each pairing, and a more defensive guy on each pairing.
Especially if they're thinking maybe they can get Henrique without having to use a 1st, I could see why they were looking at Tanev for RyJo + 1st.