Commercial Qt license is actually not required.
Qt can be obtained under an LGPL or a commercial license. The Qt company is encouraging (sometimes borderline bullying) users to purchase commercial license, but this is unnecessary for most companies. You can build a closed source commercial application using Qt’s LGPL license.
Requirements to fulfill if you use the LGPL license:
- You must make publicly available all changes that you make to Qt itself. Since you just use Qt, you would normally never need to modify Qt and so there is nothing to disclose.
- You must use Qt as a shared library (instead of statically linking it to your application). There are very rare occasions when linking mode makes significant practical difference and shared library is often the preferred linking option anyway, so it is generally very easy to fulfill this requirement.
- There are a very few optional Qt components that is distributed using commercial or GPL license (charts, virtual keyboard, etc.). They are such niche features that it is highly unlikely that you would need to use them.
- Allow users to replace Qt shared libraries. This is not an issue for desktop applications, because the files are stored on the user’s computer. On embedded systems, you need to let the user access Qt shared libraries.
- Qt company recently made long-term support releases only available for commercial Qt licensees. This is just an inconvenience: if you run into any Qt bug that bothers your users then you need to upgrade to the latest Qt version, or you need to backport the fix manually to your Qt version.
Avoiding these slight inconveniences would probably worth $10 or so per developer per month for most companies, but the Qt company charges an exorbitant $300 per developer per month for a commercial license. Therefore, many (probably most) software development companies go with the LGPL license instead.