Is this thyristor model or circuit wrong?
As far as I understand when the thyristor is triggered it should turn on and remain on unless negative voltage applied to its gate.


No, it remains on after the trigger has been removed until the anode current falls below the hold-on current which should be listed in the datasheet. Note that this value will depend on the temperature of the device.



From the datasheet.

Links


Phase Control Using Thyristors by Littlefuse.

Thyristor Theory and Design Considerations Handbook
 by ON Semiconductor.