OSS was a completely different animal back when this thing started. And it was forward thinking to avoid the whole problem with people demanding free support and bugfixes for open-source code.
As for the reasoning, a customer developing their product for years in-house and switching to commercial just before releasing would definitely place Qt financing in a shaky ground when compared to traditional commercial frameworks.
If your project decides to go commercial the initial license term will include a cost of conversion based on the quantity of code already written.
It would be interesting if they expressed that rate as something like:
A gz -1 compressed tarball of the affected source code will be created, the size (in bytes) multiplied by X is the maximum fee we would charge for retroactive licensing.
I choose the wording 'maximum' there as it still allows them to offer better deals under other circumstances.
Qt is not an end-user, boxed product. This may shock you, but B2B software doesn't always have a public, standard price list.
My rule of thumb for pricing is that if someplace /makes you ask/ they aren't going to sell at a price you'd find worthwhile.
The notion of charging by byte, as if software were content farming, shows that you are obviously not a professional.
You're right, it does sound a bit nasty. No, I haven't read the license myself. It's been brought up multiple times on official Qt blog comments and mailing lists, though.
Edit: checked it now, and it's clearly written at https://www.qt.io/faq/, "Developing with a Commercial License":
> Q: If I have started development of a project using the open source version (LGPL), can I later purchase a commercial version of Qt and move my code under that license?
> A: This is not permitted without written consent from The Qt Company. If you have already started the development with an open-source version of Qt, please contact The Qt Company to resolve the issue.