But there is zero reason to believe that any of those companies will continue giving the same returns. It's entirely feasible that they will plateau, or even be

But there is zero reason to believe that any of those companies will continue giving the same returns. It's entirely feasible that they will plateau, or even begin to fall, whereas the rest of the market will continue to climb. There are plenty of historical examples of "solid" investments that evaporated over time, so it's not like this is unprecedented.