But who is "someone like him" that can be signed out of FA? Stroman and Gausman basically? Of course there's always a consideration when making a trade on how much value you're giving up and how that stacks up to purely signing a player, but acquiring a guy like Marquez also gives the team the financial flexibility to still pursue one of these types in FA, if those exist. It's not an either or scenario. The team is high on the win curve with some key pieces of the current squad as free agents and some young, currently cheap players they'll be having to make a decision on.
Also your initial post was literally "German Marqez is a right handed Steven Matz, just a few years younger" which couldn't be further from the truth. Marquez isn't more valuable just because he's cheaper, he's legitimately easily a better pitcher.