How the thyristor in a sawtooth generator reach its holding current?
Here's my circuit: -



If I choose a generic SCR and set the hold current to be 110 mA (i.e. 10 mA above what V1 and R3 can deliver) I get this: -



Looking good!

If I set the hold current to be 70 mA, I get this: -



I think there is a great deal of justification in saying that the circuit in Albert Malvino's book: "Electronic Principles" (as portrayed in your question) is faulty. Of course I don't have that book so I can't tell you if there is some small print somewhere that says the holding current has to be above 100 mA for this circuit to work.