First, the manner in which we get said Steem is as "Steem Power," which impacts our voting strength. We're getting less of it. This means we will in turn get less of it down the line by way of curation rewards, and also that we might earn less SBD when we upvote our own posts. We likewise are able to lend less support to others through our upvotes.

One thing you have to take into account though is that the rewards pool is denominated in steem (6ish percent per year?) not in SBD so when steem's value goes up, the reward pool doesn't change in terms of steem. So your vote value in steem stays relatively the same which means that your vote value in SBD increases proportionally. So we don't get "less" steem we get the same amount but it's worth more.

In your example you start by saying that your reward is $100. This is where the mistake begins, your reward isn't $100 it's an amount of steem that is currently worth $100. So if the steem price is $1 and your reward is 100 steem then your author reward would be $100. When your post pays out, it gives you half (50 steem) in steem power and half (50 steem times current price of $1 = $50 SBD) in SBDs. But if the price goes up to $2 your reward is still 100 steem, it's now just worth $200. So when your post pays out it would give you 50 steem and 100 SBD (50 steem times $2).
This is why when you look at your pending payouts the amounts change even when the number of votes stays the same. It fluctuates based on the current price of steem.

we might earn less SBD when we upvote our own posts. We likewise are able to lend less support to others through our upvotes.

Lets say that the reward pool is 100 Steem. If steem is worth $1 than the reward pool is worth $100. Your vote affects lets say, 0.1% of the reward pool. Your vote would be worth 0.1 steem ($0.10).
Now lets say that steem price rises to $10! So now the reward pool is worth $1000 your vote is still worth .1% of the reward pool, or 0.1 steem, but now that steem is worth $1. So a higher price actually lets us all give more rewards to others and ourselves.

Your point about trying to catch up with the whales is completely true though! But other than that, a rising steem price benefits all of us.

Hopefully that makes a little sense. I tried to keep it simple.

(upvoted for visiability)