The problem is that economic growth has become the performance metric. You see the same happen at a smaller scale in companies, where a metric to measure performance ends up being the only target of the employer, instead of the actual task. For example, a call center may have a calls per day bonus, which means that most employees will be penalized for longer calls, leading them to be more pushy and cut corners, leading to customer dissatisfaction with their experience.
In order to encourage degrowth the metric has to change. It is clear that GDP is no longer a sufficient measure, because it considers neither sustainability nor equality. But without an alternative measurement to replace it economists will reject degrowth as a successful strategy because they can't see past the performance metrics they have accepted as a de facto standard.