We're looking to move one of our sites off Akamai mostly for costs reasons. Fastly configuration in Varnish VCL - Senior Engineers from my company highly rate Varnish as a cache software so Fastly was an easy choice.
I believe Akamai have been our CDN from the start. The amount of reconfiguration we'd need to do to get everything off would create a huge number of tickets in our work queue. The primary advantage of Akamai has been the number of datacenters they have to service our traffic. A customer can be in remote Australia and have their packets cached in a datacenter in the nearby telephone exchange. That's the reach of Akamai that AWS and Fastly can't compete with.
Their WAF and Bot detection products are also very good. They are a definitely an enterprise/full service CDN provider. I definitely wouldn't call them 'legacy' by any means but the type of service they provide is so different to a new player like Fastly.