Ok. By asking the state of Bukkit on Sponge you will get more then a few biased options.
Little background on me so you know where my options come from. I was rased on Bukkit. I started my programming career on it. However i always felt there was something more bukkit could do. Bukkit natively does not allow developers to create Humans (aka false players). You need to go into the Minecraft code itself to create such a thing. So when i heard about sponge I was glued to updates. When sponge api4 came about I thought it was stable enough to develop plugins on it. So I did some very small unreleased things. And at that point i fell in love with the sponge api. So much so that when api 5 came out I made the announcement that all my new public plugins will only be made for sponge (i still make bukkit plugins but they are private. And i still maintain my current public plugins with updates).
The state of Bukkit api is still very alive, with Spigot stating that they will be upgrading bukkit to 1.13. However, that update will break almost every bukkit plugin because despite them making it clear that you shouldnt use block ids and meta data a long time ago. They never gave a alternative (probably to keep compatible with older plugins. Something bukkit is known for).
I mention this because plugins that have been abandoned on bukkit will most likely not be compatible anymore. This brings the bukkit eco system down considerably. Despite this huge breakages they are not changing anything that could benefit from being changed.
It is hard to tell if old plugin developers will come back to there plugins for a 1.13 update and/or current plugins become abandoned due to the change. So the shear size of the bukkit plugin community is unknown.
Sponge on the other hand has breaking changes every main api change to suit the current minecraft code so we as sponge plugin creators are used to it and most will probably update to 1.13.
As for the quality of plugins. I tend to find a lot of basic plugins that all do the same thing on bukkit with some great exceptions that normally need updating every new minor release of minecraft. As for Sponge i tend to find a lot of different plugins that all have there uses.
As for the plugins you have. For some of them there are direct good quality ports such as worldedit and worldguard. Some that there are better alternatives for such as vault (the sponge default eco system -meaning you don’t even need a plugin). And a few that dont currently have alternatives or ports such as citizen’s.
Hope this helps
State of bukkit: active
Is bukkit dead: no but after 1.13 is lacking most plugins
Barricading yourself: didnt actually touch on. But as a development standpoint i would say you are
Plugin eco system: very different to one and another
Sponge = viable: yes - even PaperSpigot recommends Sponge over Spigot (stated in the latest state of sponge - if you haven’t watched it then you should … If you have a spare 3 hours on hand).
My recommendation: due to the plugins you are refusing to remove. Sponge is not yet ready for you. However you can go on the plugin request for sponge and ask for a plugin to be ported. Some developers (like myself will do it for free. But need to be interested in the project itself to begin with) that might help speed up the progress to you being ready.