Steem power isnt a thing. Its an estimate of the steem value of the vests youre holding.
You're not powering down steem, you're powering down vests... and the value of the vests is increasing. Because of that, your weekly powerdown will increase... If i understand the way the growth works, your last payment should be approximately 3.5 times larger than your first one.
IE, if you have 104M vests, and start a power down, youll power down 1M vests a week. Assuming you make no additional vests, you will have zero vests at the end of 104 weeks.... The increasing exchange rate will just make your weekly steem payments increase.
If youre interested, here are a couple posts i wrote about how vests, steem, etc interrelate.
yes, this is my understanding and what I wrote