This is my take too. A $10,000 purchase on 0% interest for a year earns you ~$400 in interesting from a money market fund or whatever. I'm in a high tax bracket so $160 of that goes to the government. So even for a large purchase I net $240 in a year. Hooray.
Decrease the total cost of things purchased in this manner or increase the number of different things to track (or both) and suddenly I'm paying quite a bit more in additional mental energy than I benefit in cash.
The only place where this makes sense (IMO) is when you buy a new car and can get 0% and no payments for more than a year
The nice thing about a credit card rewards program is that it requires zero mental effort. Some people like spending more effort to do churning or whatever, but I make the same computation in that case and conclude that the extra money isn't worth my time.
Paying without a credit card is just supporting the credit card fees of those who do pay with one. The price is the same, but you often get 2 or even 3% cash back (depending on offers).
Anyone financially responsible (i.e. that won't get enticed into carrying a balance by the opportunity to get cash back) is leaving money on the table by not using a credit card (with good rewards).
Decrease the total cost of things purchased in this manner or increase the number of different things to track (or both) and suddenly I'm paying quite a bit more in additional mental energy than I benefit in cash.
The only place where this makes sense (IMO) is when you buy a new car and can get 0% and no payments for more than a year