Rule 1 of most rebuilds is that prospects are kings. You do not trade any of the decent ones you have. You acquire as many as you can for your veterans as well. While the Mariners have certainly gotten prospects back in their trades, they have not been avoiding veterans.
In fact, in just about every trade they’ve made thus far, with the exception of James Paxton and Ricardo Sanchez, Seattle has acquired a veteran with some significant track record in Major League Baseball.
In these trades, they have acquired Mallex Smith, Omar Narvaez, Jay Bruce, Anthony Swarzak, Carlos Santana, and Edwin Encarnacion.