UA (and DL and AA) all got themselves into this situation - they sold miles as a great way to get free trips, pushed credit cards and big signups with that promise, and now that the supply of miles has grown significantly, the cost of actually using them for trips has finally increased accordingly. The occasional deal is still out there, but for many people (at least those who don't have super flexible schedules and have to plan trips well in advance), it's become pretty difficult to use miles for anything greater than $0.01/mile. WN/B6 offer a better value for their points for the time being; I'd personally been accumulating UR points for their flexibility, and I can always redeem them for cash. Miles, OTOH, have become a rapidly depreciating currency.
If UA wants me to accumulate miles, it has to offer value for them. But UA wants me to redeem them for $0.01/mile or less - and in that case, a regular cashback card is a far better proposition.