"Must", no. But hundreds millions of cars and trucks on the road right now can be converted to propane. It's relatively easy and well understood and you keep your same motor and drivetrain.
Converting existing cars and trucks to electric is so intensive it's almost unthinkable ... cost, energy input, waste, etc.
If the cost of this technology could be brought down within a decade, it would be a way of reducing emissions for the large chunk of the fleet that isn't electric, in a cost effective manner.
The other, more pragmatic, answer is that internal combustion engines aren't going anywhere. They have too many advantages over their competitors to ever fully go away, so we need cost effective ways of running them that are also carbon neutral or negative. Some of the links in this chain, like low cost electrolysis, are vital for other reasons (e.g. the food we grow uses fertilizer made from hydrogen split from fossil fuels. We need the fertilizer or the planet starves, and we need a source of hydrogen that doesn't also produce 11 tonnes of CO2 for every tonne of hydrogen produced).