Those charts are ancient vaping history - if all you had were dangling wick clearomizers and maybe an EVOD or two, or a cigalike 510 atomizer, then yes, follow that chart like your life depends on it.
With the older atomizers, leaving the safe zone on the chart could very well lead to excessively high temperatures. I believe this is for 3 reasons:
1) Inadequate Airflow
2) Inefficient Wicking
3) Extremely thin resistance wire (comprising the coils)
But with new rebuildable atomizers, or the new sub-ohm atomizers, all of these things are changed (or with rebuildables, you can change it yourself).
1) Airflow helps keep things cool. Cheap clearomizers tend to feel like you're sucking a watermelon through a straw, right? Not so with many equipment choices today. In fact, if I'm dry-firing my rebuildable coils, I can blow on them to keep them from glowing for an additional few seconds; the moving air really dissipates a lot of heat.
2) A well-saturated wick is probably the single most important important factor in keeping temperatures down. The battery heats the coil(s) up. This heat vaporizes the juice, which steals that heat from the coil, thereby cooling the coil. With an inefficient wick, once you've vaporized the initial juice, new liquid can't enter quickly enough to keep temperatures down; you've become a victim of the dreaded "dry hit" and all its associated nasties. But with an efficient wick, new liquid reaches the coil, continually cooling it down. You can take sustained inhales of nice, cool vapor.
3) Thin wire means heat concentrated in a smaller object. If you throw a thimble of water on the stove, it gets hot fast! But if you have a whole pot of water, it doesn't happen as quickly. It just takes more work to heat up a larger coil (and, conversely, more liquid to cool it down; efficient wicking again becomes paramount)
Simply put, there's a lot of stuff going on when you press the fire button! It's most definitely possible to have a lovely cool vape at 50 watts, because I do just that. All day, every day