Spencer and Harry Robbins saw no need to make a trade. The 190 had already been at Tweetsie for years, and was an established part of the park.
The maintenance and care that the locomotives receive at Tweetsie are second to none, and with that in mind, it's not inconceivable that Frank Coffey could expect to get 25 years out of a 70's running gear.
Frank had first hand experience with Dollywood's 70, having rebuilt the running gear at least once.