It's not so much hard as it is time consuming.

Traditional publishing, you'd usually start with a book written, find an agent, and let them shop the book around. Once you get some sales under your belt, you can actually sometimes just give your agent a summary of what you want to write, and let them sell the idea. Or just do the same with your publisher and let the agent get their cut. But you have to be a pretty reliable name for that.

Self publishing is easier, but also harder work if you want sales. You just find whatever platform you want to sell on, then put it there. But you'd have to do all the marketing yourself, and that takes a ton of time and effort. That's why agents and publishers still exist tbh

As far as adapting a public domain work (and that's what a rewrite of the inferno would be), it's totally legit. I have a retelling off Cinderella I've been fucking with for years.