I think the term "badly nerfed" is excessive in some of these cases. When you reduce the rewards potential of one of the best rewards cards in the market and it still remains one of the best rewards cards in the market, that's a pretty minor nerf. And that's exactly the case with some of these examples.
I certainly don't take these reward reductions personally. Even with the occasional reductions, we are living in a golden age of purchase rewards compared to what was available 20 years ago. CreditScholar nails the crux of the matter: any rewards structure has to be sustainable or it will necessarily change into something that is. I'm not foolish enough to expect obviously unsustainable structures to continue indefinitely.
For example, the Student Forward is probably going to remain sustainable largely because students have low incomes and tend to revolve balances. The interest (and future business, as noted above) pays for the rewards. The non-student version was less sustainable because it attracted more PIF-monthly reward-maximizing-nerd types than it did everyday folks who revolve balances.
SPG is very sustainable primarily because it is nearly impossible to transfer points in to the system, and when you can do so it's at an unfavorable rate. Consequently the vast majority of Starpoints are earned through actual stays at Starwood properties and actual spend on the SPG card. That translates into real money in the Starwood coffers which they can invest in maintaining their many excellent relationships for transfers out of the SPG system.
For the future, I think we're going to start seeing lenders get smarter and more technical about how to make their rewards programs sustainable. Less "nerfing" and more use of technical restrictions to ensure that usage of the rewards falls within sustainable guidelines.
My near-term prediction is that we'll start seeing per-transaction limits targeted towards stifling the utility of the gift cards and reload cards used in manufactured spending. For example, the Amex Mercedes Benz (not the MB Platinum), has "3x points at US gas stations for transactions of $400 or less". A lot of lenders are going to wise up to the fact that this kind of per-transaction limit makes it unprofitable to do a lot of the manufactured-spend tricks. It would not surprise me if within a year or two every 5% rewards category on every card in the market comes with this kind of restriction, and most of the 3-4% ones as well.
For the longer term, it would not surprise me to see some lenders create invitation-only rewards programs. In which you will only be invited to join a rewards structure if they have observed your spending pattern and know it will be sustainable. The ideal time to do this is at the 1-year anniversary when that waived-the-first-year annual fee is about to kick in. In order to retain the customer you review their past year of spending and offer a juicy rewards category that you know won't break the bank. For example, Chase could look back on my year with the CSP and notice I spend like a single guy, so they might say "Our anniversary reward to you is 6% back on all your grocery purchases." Meanwhile, to someone who spends like they've got a family of 5 or 6 they'd offer a different reward because 6% groceries would be untenable.
Ultimately, all these bonuses are about three things: customer acquisition, customer retention, and what share of the customer's spend goes onto your company's card(s) vs some other company's.
We are all just pawns in a cold war between lenders over who gets the biggest share of the spending from the most customers. They may change their tactics from time to time, but that's the nature of war. Change is inevitable.
The smart peasant puts out the flag of whatever army is currently putting bread on his table. 
@longtimelurker wrote:OK, "badly nerfed" really relates to your expectations.
[...]
Though I don't take it personally, I can still get annoyed.
I agree, "badly nerfed" relates to your expectations.
Would you agree that "annoyed" does, too?
I think one of the keys to both my increased happiness in life and my increased ability to maintain good credit has been recognizing that my emotions are caused more by my expectations of the world than by the things that happen to me in it.
YMMV, but for me, "I am the source of my own distress" has been a liberating way to look at the world.
--
I'm curious also about your take on "If you are into manufactured spend".
I am, to some degree, though not to the extent that some people are. If I'm buying a new Kindle, I'm going to go get an Amazon gift card at a gas station and get the 5x points from my Freedom. But I'm not trying to artificially max my spend at the 5x level on the Freedom to $1500 every month.
Even at the degree to which I'm doing manufactured spend, my mentality is that I am using the rewards program in an unsustainable way. If they were to change the program terms and say "gift card purchases are excluded from 5% rewards at gas stations" I'd think to myself "Well played, Chase" and probably apply for an Amazon Rewards card.
As I see it, if you are into manufactured spend, you are into finding unsustainable exploits of reward programs. If said rewards program finds a way to prevent those exploits, then as Omar Little would say: it's all in the game, innit?