If you create a superior api (esp w/regards to speed) people will naturally move to extensions which use that api. Then once usage of the old api has dropped significantly, you can deprecate the old api.
It has been proven time and time again, that that's at best only partially true. There are two factors:
If they have something working with one API, they have to have a good reason to re-implement it with a new API. It may be 'fast enough' and reliable, so why go through all that pain?
Or, it's just the long tail of API consumers (e.g. site on the web). Some things aren't maintained and updated, but they don't go away.
