I mean, can anyone name any big HTML5 games that make this profitable?
The Construct 2 "big HTML5 games" are listed on the homepage. Outside the bubble, I don't think there are many "big" name games made with C2. A few people have claimed to make a living from HTML5 games, but they never really offered any proof. Maybe they do.
Is there a free version that has a forced splash screen? LIke unity?
There's a free version of C2 and I think they said there will be a free version of C3. Neither will be like Unity. Why do people care about forced splash screens anyway? I haven't played a AAA game that didn't have a splash screen. Are people embarrassed of the C2 logo or something?
What are the benefits to paying this subscription VS one time fee?
The benefits to you are speculative and explained on their blog. Scirra stands to make more money with a subscription model. Users will pay every year to use their software and continue to pay if they want be able to edit and publish the games they make with C3 for the foreseeable future. Before with C2, a person could pay once for it, start a project, not touch it for a few years and then come back to it to finish it. With C3, you will have to pay for the software again.