I don't think there is anything wrong with a paid upgrade. First of all, it is kind of unrealistic to expect infinite work for a finite fee.
Secondly, you are really just complaining about their choice of name. Maybe calling it 5.0 makes you feel like you should get it for free, but they could just as easily call it "NewSlicer" and position it as a completely separate product, and it would amount to the same thing.
I have done a lot of SW licenses in my life (both as licensee and licensor), and some limitation on updates always exists. Nobody is going to sign up to give you all of the work they do for the rest of their life for free. There is always some limitation on the features, etc. that are considered within the scope of "updates", and the licensor always has the ability to create new products that aren't within that scope.
You might be thinking "but it is called Simplify5.0 and I bought 4.0, how is that not an update?" Well, I have never seen a contract that used naming as the basis for the definition of update. I can't say none exist, but if they do that is a stupid and meaningless definition. Any lawyer with a brain would reject it as the basis of defining an "update."
If this is a major upgrade, then I don't see anything inconsistent with S3D having promised "updates for life" and now charging a fee for 5.0. This simply means it isn't in scope for an update to the previous product. There is always a limit to what is considered an "update." Now if it turns out to be a handful of bugfixes, then you might have reason to complain. But since none of us have seen 5.0, I think it is premature to complain.