From the driver's perspective, the main worry is whether there is enough business to make it worthwhile to be a Grabcar driver instead being a normal taxi driver. Remember, Grabcar drivers are allowed to ferry passengers only from call bookings and not street jobs.
I think the popularity of using private cars as taxis to travel will improve when LTA address and regulate the service, reliability and security issues of this new business. When stringent LTA regulations are implemented, commuters confidence in using private cars will rocket off.
Grabcar and Uber can send taxi operators into bankruptcy if they and their drivers offer a pricing to commuters that's impossible for them to resist. For example, if Grabcar and Uber charges $20 at non-peak hours from Changi airport to Jurong Point compared to a normal cab fare of $30, which commuter would be foolish to take a normal cab.
As had happened in other countries, I guess such a day (when normal taxi drivers switch to be "private car drivers") will come sooner than expected. I can always become a GrabCar or Uber driver when that day arrives.
Good riddance COMFORT!
An interesting read: Three years on, how has Uber impacted Singapore? (Link)