..send someone to [Lapid] that will recommend him as Prime Minister with the condition that Aryeh Deri will be appointed Minister of Interior in his government, and suddenly you will discover how he comes to terms very nicely with it..

  -- Minister Zeev Elkin, responding to Lapid's criticism of the appointment of Deri as Interior Minister

I agree, Lapid would have to and would probably agree to it (if it would be an issue - it is not so clear the haredi parties would join a Lapid-led government) and his criticism is probably not honest... that being said, I think the Likud made a mistake. They are the ruling party and do not have to give in to every demand made by the smaller parties. At some point they need to, and can, say enough is enough.
Besides for that, the Ministry of the Interior was  Likud-held position, and when Silvan Shalom resigned form it, the PM could easily have appointed someone else in the Likud to that position. They did not have to give it to Deri. It was not a demand of Deri's, and when he had the chance for that seat at the beginning of the term, he chose to take the Economics Ministry rather than Interior. In Elkin's scenario, Lapid would have to give in to Deri, but in the current situation they did not have to give it to him.
It does not even make sense that they did without taking something else away from Deri. I am surprised others in the Likud let this happen without making a fuss about the proportional distribution of ministries and the fact that it was a Likud-held position and should stay that way.