I agree with you that for a downtown to thrive, it needs lots of foot traffic and people. That usually means people living downtown or very near downtown or with easy access.
You probably have not been to downtown St. Louis lately, or you would know about the loft boom going on there in the old garment district and other areas of downtown. The downtown St. Louis population is booming now, and they are starting to get the foot traffic and restaurant scene you refer to. I was there last weekend and car traffic was light, but the restaurants were packed, even with no baseball, football, or hockey in town. Centene corporation just announced it is moving its headquarters to downtown St. Louis in Ballpark Village with 2 new 30 story towers. Wachovia's just bought AG Edwards west of downtown and is moving its Richmond Securities operations there to merge with AG Edwards facilities. The downtown mall you mentioned couldn't compete with the suburbs, but it is being replaced with a large condo and office complex with retail on the first level streets. Hotels construction is everywhere and a new Four Seasons hotel high rise opens downtown in December. St. Louis aspires to having a Boston-style downtown and is getting there faster than anyone would have predicted 8 years ago.
St. Louis City got some bad PR for crime, but the Morgan Quitno "cities" report has been largely discredited by the FBI for its "cities" methodology. St. Louis City is only 12% of the metro population, and happens to contain all the metro area core crime pockets, and none of the lower crime suburbs. So the crime rate is distored compared to other cities, such as Houston, whose core crime is diluted by inclusion of suburbs in its vast city limts. In Morgan Quitno metro-area rankings, St. Louis is ranked much better -- #129, vs. #22 for Houston, I believe -- more what you would expect for a stable Midwestern city.