I suspect the problem for Western protestants is the fact that the IC was declared Dogma, not that the concept of the IC in the West is something new. Clearly in the Church the belief in the the sinlessness of the Theotokos is very, very old. Being Westerners, the idea that Panagia was conceived without Original Sin, I suspect, shouldn't cause a problem if indeed they believe in the Incarnational theology of the Church as established by the Fathers and the Councils. For the Orthodox, however, the problem would lie with two ideas. First, Orthodoxy does not accept the Western idea of Original Sin and second, of course, Orthodoxy cannot accept as "dogma" the proclamation, sua sponte, of a pope. That said, of course, Orthodoxy has always, in conformity with the belief of the people and the concesus patrum, proclaimed the sinlessness of the BVM and treated it as dogma, even if a few of the Fathers felt otherwise.
Now, given that so far as I know, Protestantism subscribes to the notion of Original Sin, I cannot see where there can be any valid argument against the dogma on the part of Western Protestants, even in light of the fact that it was proclaimed by a pope, if in fact they also accept the theology of the Incarnation. Personally, I think what we are seeing here is simple anti-Romanism, nothing deeper or more profound than that.
Twas not always thus:
"It is a sweet and pious belief that the infusion of Mary's soul was effected without original sin; so that in the very infusion of her soul she was also purified from original sin and adorned with God's gifts, receiving a pure soul infused by God; thus from the first moment she began to live she was free from all sin."
(Sermon: "On the Day of the Conception of the Mother of God," December [?] 1527; from Hartmann Grisar, S.J., Luther, authorised translation from the German by E.M. Lamond; edited by Luigi Cappadelta, London: Kegan Paul, Trench, Trubner, first edition, 1915, Vol. IV [of 6], p. 238; taken from the German Werke, Erlangen, 1826-1868, edited by J.G. Plochmann and J.A. Irmischer, 2nd ed. edited by L. Enders, Frankfurt, 1862 ff., 67 volumes; citation from 152, p. 58)
"She is full of grace, proclaimed to be entirely without sin - something exceedingly great. For God's grace fills her with everything good and makes her devoid of all evil." (Personal {"Little"} Prayer Book, 1522)
There seems to have been a "new" development in the antipathy to Mary entertained by modern Protestantism in contrast with the classical Protestantism of Luther.
Hmmmm. I'm not sure about all this use of Latin terms like 'sua sponte'. :)