Not true, as a Captain you are qualified for either seat and only a CPL is anyway required to sit in the other seat. Also because the AFS system is essentially identical from both seats there is no additional training requirement in the cruise. The aircraft is designed to be operated with the PF in their normal operating seat when below cruise altitude. Before you start blaming EK this principle comes from FARs and EASA Regs too.
You'll need a better argument than that, after all most of the time guys whine they don't get enough authority so hopping 4 feet right with a Captain in the other seat shouldn't be too onerous.