Simple correction, pagan arabs didnt just say as you quoted "lets end this animosity. we will accept what is good in your religion, you accept what is good in ours, allah, thru mohammed (SAW)"
They said " We will worship your god for few days and you worship our GODS(fire/stones/animals/etc) for few days"
How can a religion the basis and foundation of which is based on worshiping one true GOD accept and fall back on this kind of offer of worshiping creation of GOD again ?
Muslims in arabia did make many treaties of peace with non-believers but in this case the conditions were obviously unacceptable. That was the reason for the revelation of that verse.
