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Saying "strikers can be fired" can be quite hazardous (to the worker); back (decades ago) when working conditions were much worse, firing people who went on strike was common. If the only business in town, or the only place that uses your specialised skilled (like driving a Tube train) fires you, you're screwed, so you'll never complain. Down this road lies "right to work", in the style of most of the USA, and we don't want to go there.
I agree that essential workers can hold the country to ransom in the current state of affairs of being able to strike; any assumption that everyone will be reasonable is going to fail sooner or later, eg the Miners' Strike, and the current Tube strikes (and several before). I favour compulsory binding arbitration for disputes involving workers who operate infrastructure.
As to the boundary between strikes and terrorism, this is a fatuous comparison in this case: terrorism involves the use of violence, and threat of much greater violence. It's making people scared. Is anyone actually afraid of Bob Crow and his followers being violent? I don't think so. Even the NUM in the 1980s can't be described as terrorist, even if some of the did kill other (non-striking) miners such as the incident in which a concrete block was dropped onto a car from a bridge, killing the occupants.