In hypothetical world, a MILW/CRIP forced merger ca 1980 would have been a formidable competitor to BN + Frisco, even with the poor track conditions and relative "pea vine" Pacific Coast Extension and Golden State Route.
CRI&P+MILW would have basically equaled the CRI&P routes and the MILW between Chicago the Twin Cities and maybe some in South Dakota.
Just because the low point in the histories of these two railroad occurred at the same time, people like to erroneously lump them together like they were similar. So not the case. The Rock Island had lots of routes not worth keeping, that's for sure (track in Louisiana and Arkansas, and St. Louis to Kansas City as examples), but it had some really great routes that survive to this day, such as Dallas/Fort Worth to Minneapolis/St. Paul, Chicago to Omaha, and Kansas City to Santa Rosa, NM. Much of this trackage is now UP. For whatever was wrong with the Rock Island, it is proof that strong routes survive - no matter what. If a route is worth saving, it will be saved.
The Milwaukee Road is the same in that strong routes survive, so most if its didn't. Of course the main difference is that with most of its routes were considered to possess insufficient value to save, so they weren't. Were the CRI&P and MILW to merge, it is true it would have a better route from the Twin Cities or Chicago to Texas than the BN had, but that's about it, and that would entirely Rock Island trackage. The Milwaukee's Pacific Extension, the highest-cost route to the Pacific Northwest, would have remained the high cost route with an inadequate branch line network and limited access to Canada even with the Rock Island as part of the system. BN would still have the superior route from the Pacific Northwest to the Midwest, and the MILW-CRI&P routing from, say, Portland to Dallas would be an exercise in circuity. Basically when you look at the shortcomings of such a system against the BN, just about all of them are associated with the Milwaukee Road.
The Rock Island and Milwaukee Road perhaps had similarities with regard to the why of bankruptcy in the 1970s, but the value of their route structure was not similar.