The Church of the 16th Century was in desperate need of reforming. Lots of people said so, from all over the Christian spectrum. And they came so very close to actually reforming the Church without splitting it. But in the end, the papal party refused.
Indeed, the Reformation was a three-handed game (at least). Those who eventually became known as Protestant and many of those who stayed Catholic could have made common cause. But an older contest was still being played out: Popes vs. Councils. By a brilliant bit of triangulation, the papal party managed to make the issue Protestants vs. Catholics, and the conciliarists caved.
In order to do this, the Pope had to move the Roman Catholic Church at least as far in one way as the Protestants moved their Churches in the other. So to say that the Protestants either "left" or were "expelled" is less accurate than to say that all the pieces of the western Church moved away from each other, to the injury of all. And all the leaders of the various fragments bear responsibility for the division that resulted.
But -- was not the Church in need of reforming? Yes. And is it not still in need of reforming today? Oh, yes, indeed. The results achieved by the Reformers were a mixed bag, no doubt. The results achieved by reformers today will be much the same. But still, the Church is need of reforming.
So today we honor some of those who attempted to save the Church from itself. Some were perhaps too eager to tear off a piece and make it what they thought it should be; meanwhile, others tried long and hard to preserve the Church's unity while nevertheless cleaning it up. Sometimes there were no good choices and people who loved Christ and loved each other as brothers in Christ found themselves on opposite sides. But at least they tried to make the Church better, and were not content to turn a blind eye to corruption and scandal.
May their tribe increase.