Taxi cab companies with company owned fleets often have central garages for repair, refueling, and driver switching. It would also be sensible to locate service facilities near airports and other areas where taxis frequent. Metro taxi companies also use natural gas and have their own facilities for refueling.
Yet natural gas/propane based cab companies are very rare. That should tell you something about the economic value of using stock vehicles that can be refueled anywhere. Uber's entire business model is built on using stock vehicles.
Actually taxi companies are far more interested in operating costs than buying stock vehicles. It has always been that way.
Capital costs cannot be ignored because cars depreciate and need replacing. That said, the biggest cost for cab companies is down time. A car not carrying paying passengers is losing money. Companies that can minimize that will do better. A company could invest in all of the facilities to keep a fleet of EVs running only to find that a competitor with ICE vehicles is doing better because of more running time.
Of course, running a taxi company in most cities is more about crony capitalism than the free market. So I am sure some cities will simply mandate EVs cabs, ban Uber and force consumers to pay.