This makes a couple assumptions that may or may not be true.
1. Arenado has only confirmed 6 teams he’s willing to play for but his agent suggests the list is much longer. So there could be lots of other options.
2. The Cardinals have said Contreras will be 1B/DH but haven’t said anything about Burleson. Burleson may still be the primary at 1B against right handed pitching. In this scenario, Contreras is the primary DH.
I do think the Seattle trade you proposed is interesting though.