The Catalyst may survive, but it has achieved it's purpose. It, the Reapers, and all other Synthetic life are now at peace with organics in this Galaxy. Not enslavement at all; free willed Synthesized life forms working in unison for a future peace.
The people who were used to make the Reapers were subjected to it involuntarily. Each Reaper's "mind" is formed from a combination of A.I. processes and the millions of organic minds used to create it, all bound into a kind of gestalt intelligence.
That is slavery, any way you slice it, as the people used to create the Reapers were stripped off their free will and individuality, each bound to millions of other victims and A.I. programming to form a single Reaper's brain. Each Reaper is in turn bound to every other Reaper, and to the Catalyst in a master-slave relationship.
There's nothing to indicate the shackles that bind Reapers to the Catalyst are removed in the aftermath of Synthesis. Can that even be done? The Catalyst makes it clear that he and the Reapers are not entirely separate entities, as their minds are all linked similar to the Geth. Even if it could, it would not change that each individual person used to make a Reaper was still bound to millions of other organic minds and an artificial intelligence to create a single Reaper hive mind. It's horrific, and Synthesis allows it to continue.
Where is it shown that the catalyst wants the reaper survival ?
The epilogue for Synthesis where the Reapers are explicitly shown to be very much alive and kicking.