No, they are betting that no matter the processing power of computing devices of the day, developers will always push things to the limit and ship products that can end up being slow on devices used by a large share of the population.
This has always happened, because software product makers always want their products to have as many features/capabilities as possible, and release them to market as quickly/cheaply as possible, which means there are lesser incentives to limit functionality or to invest in performance optimisation.
There's no reason to believe this trend will cease, so Mighty is offering a service that allows people to get much higher performance of their webapps without always needing to have the highest-power computing device in their possession.