Rather than a Grand Scheme, I personally feel it is a plot hole, like why called it 10000 years plan. If he found the possibility of planet fall, then he should know one of the factor is the spirit beast decline, yet he didn't show any plan at all. Also why did there is so many restrictions with the God classification and Super Divine weapon, since Sage King manage to create it then he should be able to use it at full capability. The way I see is just an excuse so the grandparents has a way to overcome Sage King