The sign up bonuses on cashback cards are simply too low to bother with them. If as a generic middle/upper middle class couple you each sign up for 4 cards a year, and cross refer, hardly an aggressive strategy, you will have enough minimum spends to meet to swallow most of your credit card spending per yer, and you should end up with enough miles to get at least 3 RT trips every 2 years in premium cabins overseas. At 4 cards a year each you are unlikely to run out of cards for a fair while, even with the current restrictions. Even non aggressive card cycling, especially in two player mode, provides seriously good returns per dollar. Cashback, unless you spend serious $$$ cannot do that. 3 holidays with your flights in J is a massive upgrade (!). Yes if your work always buys you J tickets for overseas travel its not a massive deal but for the mere mortals it is major value.
Travel rewards won't die unless simultaneously all carriers move their programs to fixed or fixed but with a slight range value per point, and the Sign up bonuses dramatically decrease, by a factor of 5 at least. Only then will cashback be the only game in town.