Those defenses collapsed at least as often before the offense had a chance to fail. Once the early talent moved on, those defenses were indefensible.
There should be a distinction between the early and late Rhoads eras.
Early Rhoads -- the one who actually did a pretty good job by the historical standards of Iowa State football in making three out of four bowl games and just missing against Nebraska for 4/4 -- had teams with very good defenses and decent-ish offenses. Nothing flashy, but they scored enough. Obviously, they were no threat to win the Big 12 and were happy to go 6-6 or 7-5 with maybe one signature upset in there, but again, that was really good by our historical standards.
Late Rhoads was just awful on both fronts. Some of the "middle" Rhoads defenses would try to keep the team in games before they ground down, but by the end just everything in the program was malfunctioning.
Then Campbell came along and blew all our expectations completely out of the water.
I feel like a lot of people pushing back on assertions that Rhoads had some good teams that had some good accomplishments by our meager standards only want to worry about how it ended, not how it started, because it started pretty good with those bowl games and some upsets we should still all cherish (TX, NE, OSU, etc.).