What would Detroit look like if the RenCen hadn't been built?
Do you think we have eventually had 5 towers of comparable size to the 5 RenCen towers built in different places north of Jefferson?
Would Comerica Tower's planned twin have gotten built?
Would our Riverfront currently be more of a park-like riverfront like Windsor has?
How's this for alternate reality?
If the Renaissance Center had not been built, GM likely would have moved out of Detroit in 1996, this place would be in even more horrific shape than it is now, and the old GM building would be vacant. The availability of the Renaissance Center probably prevented the demolition of the old GM facility - because the availability of cheap office space enabled GM to do the redevelopment deal. With no RenCen, the Millender Center site likely would still be a dirt lot.
People love to hate the Renaissance Center, but few buildings better evoke the ethos of post-riot Detroit [[or of 1970s urban America in general). The use of beige tile and unfinished polymer-impregnated cement is pure John Portman. Go to Embarcadero Center in San Francisco or Peachtree Center in Atlanta. I suspect that in 20 years, people will be complaining about how the building was "wrecked" by the SmithGroup redesigns of the front and back of the building and the circulation ring.
In terms of what was lost in the building of the Renaissance Center, the train capability was not taken out until 500 and 600 were built. There are pictures of the commuter trains on Franklin in front of the building [[or to the east of it), and Cadillac used them in a recent ad campaign. I think they are Tony Spina shots, but I'm not 100% sure. The Robin Hood flour mill was an eyesore and a very poor use of riverfront land. My suspicion is that the siting of the Renaissance Center was so that it would be visible down the major road corridors, and it just happened to eliminate one particularly ugly riverfront use.
The effect on the rest of downtown? Probably a lot less than you might think. As far as I can tell, Detroit had no other new Class A office space built between 1970 and the mid-1980s, and Southfield was building it like crazy. GM's move into the Renaissance Center may have driven some tenants to the suburbs, but the RenCen was probably the only thing that had kept them in Detroit in the first place. With a declining office population, no nighttime population, and poorly maintained older office buildings, downtown in the 1970s-1980s was, to put it politely, challenged. The RenCen was supremely functional as office space, and it had spectacular views.
Consider also that Henry Ford II [[who conceived the RenCen) also used Ford's purchasing power to strongarm a lot of companies to invest in and locate in the complex. Those people weren't going to move into any of the existing office buildings downtown.