As Kallis says, the reasoning you have put in your post seems perfectly valid. Yes, the creditors are getting less, on paper ---- they ( and the IP) will save a fair chunk on 4 years administration fees as well. There is every chance that they will sell the debt on during the term for considerably less, as well as the chance, due to the uncertainties you outlined, that the IVA will fail and they will then get less. "A bird in the hand is worth two in the bush".
That all said, it is not in gift of your IP to second guess what the creditors will decide, this is up to your creditors to weigh the pros and cons.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014