Affordable, but a little hard to justify as a casual user who just wants to mess around and doesn't really plan on building anything complete

It’s completely free for the casual user who just wants to mess around, as long as they don’t use it for commercial work. Almost all features supported, except external renderers and certain geometry exports.

There are some great free tutorials here: https://entagma.com/

It's not the price that's the problem. It's that it is closed source, and therefore any work you produce is on a guaranteed path to bitrot.

Isn't the final rendered product portable? Also, I'd imagine you could always export the necessary assets into a format less prone to bitrot.

I don't know of any format that supports full parametric modeling the way Houdini does.

As the OP said, parametric modelling is what makes Houdini so nice.

And if you want any chance to rework anything in a project - say - 10 years later, exporting to a "baked" format is a dead end.

What's preventing the development of an open model/data format for parametric modelling?

Because every feature and parameter would have to be baked into the format and every editor would have to offer those same features and no more.

There's always a trade-off between editability and fidelity with any format. Non-destructive stacks of modifiers bring that trade-off into sharp relief.

Yes.. not only offer the exact same features but any software that tries to implement the format would have to agree on the exact algorithms for the transformations represented by each procedural step. Any error in any one of those algorithms would cascade into subsequent steps and could generate a completely different final product

Some amount of variation is desirable in any artistic endeavor.