It is not theft. We have been through three versions of the Guild Dethrone system.
- Vanilla: After 30 days inactive a guild member could put in a ticket to dethrone an inactive Guild Master.
- Later, they automated that so that if a Guild Master was inactive for 30 days or more the leadership was automatically offered to other guild members starting at the highest rank and working down to the lowest. There were a lot of complaints about that version.
- Current version: Guild Master is offline for more than 90 days it gets offered to the top 3 ranks in order. The GM can set it up so that anyone else is not within the rank levels to ever get it and can retain the guild forever. This has been in place for many years now… Here is a 2018 guidance article about it in addition to the Support articles Spotlight On - Guild Management and Security
What you might have thought was rank 8 then, was not. He was within the ranks to get it offered to him, and took it. That is how the system is supposed to work. It is up to the GM to set up ranks, bank permissions, etc.
Blizzard CS will not get involved because the system is working as designed. It is designed to have active leadership for a social group, which is what Guilds are intended to be. Of course some of us have bank guilds or small social alt guilds, which we have to be careful how we set up to avoid possible dethrone.
Your option is to start a new guild really.