Gradually Stake may increase the requirement to diamond VIP rank, who knows! Now the requirement for getting a VIP host is Platinum 4 VIP rank, 2.5 million dollars wagering is required to reach that rank.
That means stake are friendly to the whales not with small gamblers we won't get a chance to have a VIP host we hope the same doesn't happen to the bonus system because this is one of the reasons we stay on stake. I suspect stake did this because of the increase in new VIP members so there is no other way stakes have to tighten the rules.
Yes I believe it is because the increasing number of Platinum users is beyond their expectation.
On the other side, they have limited number of VIP host while it is not easy to hire people that can be trusted and educated to be VIP host.
I got VIP host when I was ranked up to Platinum 1 few years back and I still have it till now, so lucky me to start earlier
If the number of Platinum users (1-5) is increased significantly but Stake is unable to add more VIP hosts, the requirement might be increased again as what Mahdikarib said.