Well, it was pretty common to see Mantua and Athearn rolling stock on layouts at the time. I wouldn’t consider them “toys” in the way that I would consider the Tyco Purina box car to be a “toy.”
Respectfully, I do feel that the switch from metal to plastic represented a decline in quality, both in terms of weight of the cars and detail.
In terms of older Mantua rolling stock vs. the Classic line, the classics have plastic bases and plastic trucks. In the case of the gondola, you cannot even place the Mantual Classic body onto the underframe of an older model.
The Mantua Classic clearance check car has plastic trucks, and includes only two of the underside wire brace rods instead of four.
My point is that, given the name of the Mantua Classics line, one might have assumed that these were either accurate repros of the old cars, or an upgrade. Of the cars I’ve seen, they were cheaper.