It would have absolutely been the right play imo. The free agent pool has been thinning significantly as quality options re-sign with their teams. Next year looks like Marner (who Leafs can outspend anyone if they choose); and Florida Man Sam Bennett and Ekblad.
https://www.spotrac.com/nhl/free-agents/_/year/2025/sort/contract_value
The Oilers continue to dip into their savings account with draft picks and a very poor prospect pool. They will likely have to spend their way into creating a sustaining window with a high growth cap going up over $20+ million over the next three-four years. Rantanen is an elite prime years guy who would have moved the needle immediately and quite likely has a favourable aging curve.
If teams smartly continue to lock up their top talent with cap requirements needing all teams to spend, it might be a wee bit more challenging for the Oil to poach via the inefficient, expensive free agent route. And they've already been challenged this trade deadline from accessing high end guys like Rantanen and others because they have way more limited pedigree assets to throw into these competitive bidding processes.