Marriott Bonvoy points can be used to transfer them to a variety of different airline partners. The transfer ratio is usually Marriott points to airline miles. If you transfer 60,000 points, you will get 25,000 miles instead of 20,000, making the transfer ratio 3 to 1.25, a much better deal and something that can be useful for those that are awash in Marriott points.
The 5,000 miles conversion bonus with American, Delta and Avianca will no longer be offered by Marriott. The bonus is still intact for the other 35 partners.
You can find Marriott's full list of airline transfer partners here.
I only took advantage of Marriott's points-to-miles transfer once, as it's more difficult for me to efficiently generate their points compared to other companies. As Bonvoy award prices climb, there is a significant chunk of people that linger with 7- and 8-figure balances.
The announcement doesn't affect the points-to-miles transfers as a whole, but it does affect the value of two of the three biggest domestic partners. Delta and American were two of the more popular destinations and Marriott doesn't feel the need to encourage their use anymore.
The devaluation came as a bit of a surprise and I think it is a sign of things to come.