The bottom-up approach
The corollary to top-down is "bottom-up", in other words, taking a mainstream product and pushing the boundaries of the design to include as many potential users as possible. Bottom-up approaches aim to take a mainstream product that is to be designed for able-bodied users and make it more inclusive.
Bottom-up approaches offer great potential for commercially successful products. Starting at the bottom of the user pyramid and expanding upwards means large numbers of people are included by the product. However, it must be noted that for each unit move up the pyramid, the additional number of people included diminishes. Consequently one of the drawbacks of the bottom-up approach is that companies may be tempted to settle for the able-bodied market only and ignore what they perceive as a diminishing return when attempting to move up the pyramid, especially if the cost and effort involved are significant.
Another difficulty with bottom-up approaches is that they can only go a limited distance up the pyramid. There will still be users who cannot access or use the product and may require something radically different to be designed (someone with no hands will not be able to use any form of mainstream kettle to make hot drink). Thus it is highly probable that bottom-up approaches will always need to be complemented by top-down ones to provide coverage of the needs of the whole population.