Oddly, urban economics was not emphasized -- even though Bill Alonso's book was based on the first dissertation out of Penn's Regional Science program. But Isard was an enthusiastic big-tent guy and would never exclude anything from his vision of Regional Science. It was the 1960s and most social scientists were optimistic. Isard was eager to develop tools of analysis that sophisticated regional planners and policy makers could utilize.
I do not know how long Walter held this view. But he was an enthusiast.
I now lean to the view that cities are spontaneous orders. The New Urbanist view that they are the purview of planners, it seems to me, is a Fatal Conceit. Sandy Ikeda and I have a paper (forthcoming) that speculates on how the insights of Jane Jacobs and F.A. Hayek mesh in this regard. But there are many loose ends. Randall Holcombe has an insightful paper on these problems wherein he suggests a division of labor between markets and planners. The latter do the best infrastructure planning they can and let land markets do what they do best.
The problem is that infrastructure planning is usually anchored to some view of what the land use arrangements are or will be. This is OK in a static world, but very difficult in a dynamic context. Urban designers can invoke a dozen cliches (put office high rise near transit station), but these fall way short. Real life complexity is another galaxy.
English speakers learn about a city by consulting a city map. In many other languages, these are referred to as the "city plan". People get a document that can be construed as a plan only in a static world.
Here is more abut Walter Isard. A full life.