Holding Current and Latching Current dilemma
For the model you have there the holding current would be extremely small. But it is not a perfect model.
Imagine for example modifying the model with a resistor between the gate and the cathode to represent the leakage current, say 1kilohm. If the current was less than about 500uA there would not be enough current into the base of the lower transistor to sustain regeneration and the SCR would turn off.
Also, the gain of the transistors (hFE) varies with the current which affects the holding current.