The official WP line is that if you want to share sell or distribute your plugin/theme, then the PHP code must be GPL compatible. This is because GPL is a copyleft licence. The WP project, maintainers, and governance follow the same interpretation of this that the FSF and others do, the same interpretation being used to sue the TV maker Vizio in court for not distributing source despite using opensource libraries.
Some people may disagree with this interpretation, but that is a legal subject and beyond the scope of this site. I am not a lawyer and it is for the judge in that case to set the precedent.
Your other points though, are covered by the FAQ on the GNU website:
https://www.gnu.org/licenses/gpl-faq.html
The GPL does not prevent you from selling themes and plugins, and going non-GPL does not prevent piracy or provide other protections ( and it’s questionable that a “proprietary” licence would even be valid or enforceable ). The biggest businesses in the WP ecosystem still make plenty of money with GPL source code.
What I can say is that:
- Your plugin will never be allowed on wordpress.org
- Other marketplaces will probably not host you
- Other plugin businesses in the WP ecosystem will not want to do business with you, limiting your ability to promote and cross-sell
- Applications to speak or volunteer at official WordPress conferences will be blocked
- Sponsoring official WordPress events will be impossible
- Many prominent community members will see it as a hostile and unnecessary step
- Many large agencies will avoid your product
- Larger enterprise businesses will have compliance issues around this
- Some hosts may refuse to allow your code
- Many will consider it a sign of shady practices
- Some businesses that have made a point of not using GPL compatible licenses have had… not so pleasant interactions with the BDFL
If despite this, you still wish to pursue a proprietary licence that is not compatible with the GPL, and distribute this, you should seek legal advice from a lawyer specialising in these matters. You shouldn’t be asking for legal advice online.