Adaption and experience turns that sharp cliff of bonking into a gentle slope, but you're going down eventually nonetheless. Five hours matches my experience as well and is also roughly the duration of a half-ironman.
But you can do both. Water for everything up to a longer (but not the longest) Sunday ride, carbs for multi-day (you won't replenish glycogen in one night), for true all day riding and for the occasional short all out competition.
What you can't do is run on water all the time, then suddenly add carbs and expect it to work out well: when I started venturing beyond my "water only horizon" (which was more like three hours at the time) it felt very much as if the body would shut down all other energy paths once carbs were available and won't be able to restore them once the carbs run out and then you need to refuel as fast as a carbs-only athlete and that's just impossible for someone used to water.
Now I think that a more accurate model is that when carbs become available, an athlete not used to that will increase energy throughput by more than what the extra carbs can supply and exhaust the other sources during the sugar high. After that it is impossible to apply carbs fast enough to compensate. But avoiding overexertion during the sugar high is easy to learn and then you are an energy generalist, going for hours when water will be enough and adding some carbs on those special occasions.
But you can do both. Water for everything up to a longer (but not the longest) Sunday ride, carbs for multi-day (you won't replenish glycogen in one night), for true all day riding and for the occasional short all out competition.
What you can't do is run on water all the time, then suddenly add carbs and expect it to work out well: when I started venturing beyond my "water only horizon" (which was more like three hours at the time) it felt very much as if the body would shut down all other energy paths once carbs were available and won't be able to restore them once the carbs run out and then you need to refuel as fast as a carbs-only athlete and that's just impossible for someone used to water.
Now I think that a more accurate model is that when carbs become available, an athlete not used to that will increase energy throughput by more than what the extra carbs can supply and exhaust the other sources during the sugar high. After that it is impossible to apply carbs fast enough to compensate. But avoiding overexertion during the sugar high is easy to learn and then you are an energy generalist, going for hours when water will be enough and adding some carbs on those special occasions.