Yes, I think you are right. However, I can't explain their intention of making it more complex to become a VIP but by the rising number of players.
I think it's normal if they make it more complex and it seems very difficult to become a VIP on their site. Maybe they need to make this change because looking back, maybe people could easily reach the VIP level so Stake had a lot of VIPs while they wanted to give something more to their VIP members.
So maybe they select their members more and only choose people who are really loyal to them to be their VIP members.
There's a tier system on VIP so they don't need to become strict on the general terms because they can always increase the tier and lower the reward of low level VIP if they really want to only benefit players that playing big amount in there website. I think there move is to push players to play more to get the VIP bonus since they giving a way good amount as reward in all there VIP tiers. Stake is known for being a generous Casino in terms of rewarding there players, I'm sure they have a good reason why they do it.