[White Genesis]

I'll go with the Turbo powers being the reason Billy was vanishing when the Gold Ranger first appeared but in my headcanon, they weren't meant to replace Zeo at first. They were meant to be a set of powers for a team Billy was going to form himself when Tommy and co retired because the Zeo Powers couldn't be easily transferred. When Jason came close to dying from the Gold zeo powers, Billy rushed to complete them and left them to be used later after leaving Earth.

After Zeo, Jason left because he no longer felt needed and Zedd and Rita made a comback in a smaller, yet more leathal version of Serpenterra. The fight pushes the Zeo powers to the limits and severely drains the rangers to the point that morphing must be kept at a minimum. Zedd and Rita decide to semi-retire, give birth to Thrax. age him to adulthood to get out of raising him, and ship him off with his grandpa and Uncle Rito. He acts as Master Vile's warlord for a time but is banished before the Z Wave strikes and travels the universe until he comes across Sentinel knight. Also, the Machine Empire is humbled rejoins the United Alliance of Evil after Dark Specter rebuilds them.