Disruptive Innovation does not necessarily start at the bottom. It is about focusing on a new metric for performance rather than improving an existing one that has become over-addressed by incumbents.
The new axis of performance allows for new applications or new markets to open up, despite poor initial competitive position vs incumbents. Over time, performance along traditional metrics can become good enough for the new technology to directly compete with incumbents.
As an example from The Innovator’s Dilemma, solid state disk storage is not a “low end” solution. It offers smaller size and a resistance to damage from shaking that makes it great for iPods and smart phones. But it is very expensive and initially seemed like a posh luxury product.
When small computers started to catch on, traditional disk manufacturers were focused on dollars-per-gigabyte, which solid state storage does not compete well in (the traditional competitive metric) but solid state storage could do things that spinning platter disks were bad at (such as: keep working when you drop them on the floor).
Over time, solid state storage has come down in price to the point where it can compete with traditional platter disks because the price is “low enough” and the speed and shake-resistance benefits make it appealing, especially for laptops.
This last bit is the Disruption, of course. In this example, it’s Disruption from a “premium” technology rather than from a “discount” product (an example of the latter from The Innovator’s Dilemma would be mini-mill steel).