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You need to complain to the plugin devs that aren't porting over to sponge or forge.
Some of us even ported bukkit plugins and/or make our own plugins for forge. If your need for all your plugins is critical to your server, i would stay where you are. pixelmon 4.0 and mc 1.8 are still giving hell when it comes to running a server.
Sponge is still in development, i know a few of us have given up and removed it until it becomes more stable/useful. But if you still want to try your luck, you can read up on all things sponge here: https://docs.spongepowered.org/en/
Good luck. Jay113355