I would argue that Ekholm is simply better than Andersson but that is besides the point. The reality is that Ekholm was not a rental. In fact, I think that the generic price of a rental is being significantly overstated with your comment.
And I will say again, I don't see a trade for Andersson as being at all in the cards.
Look at the offer. The Oilers' surge up the standings has devalued the pick. It's now a contender's 1st instead of a bubble 1st. Broberg is not an NHL regular D+5. He's a reclamation project, not a notable trade asset, imo. In the trade, Ceci is a neutral value cap balancer with that extra year; the point would be for Calgary to extract value at the deadline in 2025. Savoie is more contract slot than prospect at this time.
Here are the top defense rentals in recent years:
Savard - Columbus recieved a 1st and 3rd
Chiarot - 1st + 4th + B-prospect (recent 3rd)
Lindholm (Trade & Sign) - 1st + 2 2nds + Vaakanainen; Anaheim took back 50% of a John Moore multi-year cap dump in addition to the retention
Orlov - Traded with Hathaway, returning a 1st + 2nd + 3rd; Washington took back an expiring Craig Smith as a cap balancer in addition to the retention
Gavrikov - Traded with Korpisalo, returning a conditional 1st (2 2nds if the Kings missed the playoffs) and 3rd; Columbus had to take back Quick as a cap balancer
...
Andersson is certaintly in that class of player, partciularly with his handedness. A 1st and an early Day 2 pick is a given in this era.