A project is a collaborative work of multiple users, not only the project admin. To prevent the project admin from destroying the collaborative work, I thought, it would be better to prevent this.

I see the practical problem with this. We could give the project admin the right to disable a project. Disabled projects would be invisible to users. The system admin could then delete it, or configure a delayed (14 days) auto-delete for disabled projects.

What do you think?

Thanks for answer. I would agree with you. Disabling instead of immidiate delete is better idea.
Delayed Auto-delete should be an option in System configuration. In my opinion auto-delete should be disabled by default.

Nevertheless, I think that deleting by project admin should be possible if there would be no content in project provided by other participants (there would be no collaborative work). The example here would be creating project "by mistake" or when project admin would change his mind after configuring project.

I also think that after project admin would mark project as disabled it would be useful to send email to all participants with proper notification.

Disabled projects should be accessible (read only) for all project participants. They might want to copy some content from Collaboration project part or just see project history to draw some conclusions.

What is more, from my expirience there are sometimes projects that are just suspended for some time - often example - internal projects. Which need to be resumed when there are circumstances to do that and continue work in project. So I would think about creating functionality to "Enable" disabled projects by project admin which would just take off readonly constrains and send proper notification to participants.

I have just checked that System admin can not reassign project admin role - this may be needed if there would be need to change that person (i.e. the person is dead by accident). Just my thoughts.