To be honest at the risk of causing someone else to be upset, the words “schismatics and heretics” are the words St. Faustina reported that Jesus said to her. If the Church is going to promote a devotion that it believes came from Jesus himself, then I would be loathe to change the words that Jesus is reported to have said, and not just dismiss them as “Jesus was speaking to St. Faustina in the context of the culture of her time” or whatever.
I also believe that if people are going to up and leave the Church because they feel that strongly about their convictions of conscience or whatever that it’s the right thing to do, they should not be upset if the Church calls them a schismatic or a heretic. I would think they’d say, “If being correct (in their opinion, they are correct) means being a heretic according to your Church, then so be it.” And finally, I think the “politically correct” change is a bit two-faced in that if one understands the Catholic perspective, we’re still pretty much thinking Protestants are “separated brethren”. I understand there is now even an objection to the term “separated brethren”.
These are just my thoughts, however. I tend to come down on the blunt side when it comes to Protestants and SSPX and other groups that leave the Church. Bridge-building in this context is not my strong point. The Church officials in charge who changed the prayer obviously see things different from me, and I defer to their judgment.