When to invest in on-demand
The “on-demand” fit of the product and market depends on a few things.
An exposition
There are certain sectors in which the on-demand economy would actually work. These would have to be services which come with a sense of urgency built into the decision making heuristic.
The “value trap”

High urgency, low volume businesses (like an ambulance) can be made on-demand but might have a cost structure that is not feasible for surge pricing. An Uber for Ambulances will almost always be a surge pricing ride if we go by the regular definition. Similar examples : Uber for Financial Advice would be a flop because no one wants financial advice in the next 12 minutes, and so on. This framework can be a handy guide to choose whether to invest in a given on-demand service or not.