I feel like this shouldn't be surprising. If you use GPL, you give others strong incentive to rewrite an alternative, because GPL is just very heavy burden. I'd even go as far as to say this is an example of failure of GPL. If the intent was to set up incentives to make sure people share the improvements with the community for mutual benefit, but the result is an incentive to rather burn resources to do complete rewrites, then that seems like a failure on all sides to me.