You're completely right. Hashpower between chains is only relevant if they are compatible (same consensus rules), as the accumulated proof of work is used as a

You're completely right. Hashpower between chains is only relevant if they are compatible (same consensus rules), as the accumulated proof of work is used as an indicator of which chain should be followed. Once a chain hard forks to alter the consensus rules like Dogecoin, Bitcoin Cash, etc, there is no amount of additional hashpower that would make up for the incompatibility -- a node will simply ignore the invalid blocks.