Steen trade:
St Louis would essentially be moving down to clear almost 6 million in cap. This could be worth it if they need extra cap to bring back Pietrangelo, Dunn, etc.
Detroit would be adding an extra 1st this year to take on cap. They would likely have to agree to buyout Steen to get him to waive his no-trade clause but it only costs about 3.5 million for next season and about 1 million the year after.
Allen trade:
St Louis may choose to keep Allen as Binnington didn't look great during the playoffs this year but it could hurt them to spend over 4 million on a backup goalie.
Many of Detroit's realistic free agent goalie targets have played/are playing well during the playoffs (Talbot/Khudobin) so their teams could resign them forcing Detroit to look to the trade market for a second goalie. Allen played great last season behind a strong St Louis team but would need to transition to playing behind one of the worst teams in the league. However, he could have the potential to be our main goalie for at least a few years before we become competitive again, and worth the price of a mid-round pick.