Until recently, you could argue that if someone frum was in one of the frum units, or was being very stubborn, they would have no problem in the army, and it would be permissable to serve just as one sometimes needs to take job in the goyish world for parnassah.
But not that the medinah has embarked on a policy of persecution and coercion aimed at “breaking” the yeshiva world, one can argue it has a full status of “Sha’as ha-Shmad” and we have to resist in all possible ways.
In all fairness, there are still many ways they could back down and perhaps limit the maximum sanction to losing any stipends paid to yeshiva students, but not cancelling civil rights or welfare state benefits, and for political reasons, neither Likud not Bayit Yehudi wants to “burn bridges” to the hareidim. However if they start to throw people in jail for not paying fines, or seizing money raised privately to support learning – the situation will seriously deteriorate, perhaps to the point where some secular bigots can accomplish what all the terrorists have failed to do – and bring down the medinah once and for all.