2 things..
First. A cell tower issue can, in fact, be a one time thing. Antenna's, RF lines, BTS's, Telco etc.. all components of a cell tower can go bad or suffer issues. If your car burns out a washer fluid pump does that mean your car is "bad" forever more? No.. It means you fix it. The same for a cell tower and all it's associated equipment.
Second. Sprint and Verizon don't use the same towers. This is oversimplification of a larger issue. Sprint's traffic will never run on a Verizon tower unless there isn't a Sprint tower in the area. And then ONLY if they have a roaming agreement in that area.
Carriers do frequently co-locate on the same towers. But everyone has their own equipment on that tower. Their own antennas, their own RF lines, their own BTS's. There are multiple factors that can influence carriers. So to say it's all CDMA and therefore the same is really an ignorant statement (ignorant in an uninformed way and not an insulting one). You can put 4 CDMA carriers on one tower and I can show you 4 different footprints, 4 different spectrum useages, 4 different tolerances for noise, 4 different capacity builds and 4 different ways to build a network. Even something as simple as antenna position on a tower will effect things. Cricket? Cricket operates so high in the CDMA spectrum that, at the same power output, they have almost 20% less coverage than another carrier lower in the spectrum range.
Advertising is snake oil and who can peddle words the best. National carriers will almost ALWAYS include the words "National" or "Nationwide" in their pitches. Why? Because they can't say they have the best network period as there are many rural and mom and pop carriers who can absolutely crush them in network quality.. in their areas. But they aren't nationwide.
5 years as a switch engineer and 10 as a Network Field Engineer for a Wireless carrier back me up on this.