Whatever beliefs Luther and Calvin may have shared, they also differed in many respects. Luther called himself and his followers Evangelicals. I don't believe Calvin could ever be mistaken for an Evangelical Lutheran.
Over time (it wasn't long) heresy crept in with Joseph Arminius and his free will views. The TULIP was developed to counteract the heresy of the Remonstrant which mirrors Catholic doctrine.
You are correct that there is some significant overlap between what most Protestants believe and Catholics believe on certain issues. As for the idea of free will, many Protestants looked at the issue and ended up agreeing with the Catholics on this issue rather than Calvin. They didn't read Arminius first and then conclude that he was right. Arminius was simply the first we have a record of who came to this conclusion, so his name is used to describe this position among Protestants.
There are precious few Protestants that are Reformed holding more of a Catholic version of man's free will.
We all write quickly on these discussion boards without editing. You may want to rephrase that sentence because I think you said the opposite of what you wanted to say.
Today there are precious few Reformed Protestants. Reformers believe that God saves people through His grace alone. Most Protestants hold a Catholic version of man's free will requiring man to "cooperate" or to have "faith". It this latter instance, many have rejected the notion of sola gratia.