I realize I'm new to CoTP, but I do have been playing Minecraft multiplayer since before monsters were even implemented on multiplayer servers. I've been through many different servers, some have gone and some have stayed. What I have basically seen is that: You will have drops in numbers at times and rises in numbers at times. Often they occur more so based on time of year and not as much based on actually playability of minecraft. For instance on one public server I'm on, during the month of April due to most college aged players having finals the numbers dipped down substantially but as finals have been over the numbers have increased substantially. Also time of day can often have a big effect.
Again I understand I'm new to the guild so take that with a grain of salt but I personally do not like tekkit servers because even though you don't have creative powers you are able to set up those mining tools which give you all sorts of stuff. Yeah it's sorta hard to make them but it does take away a good portion of vanilla in that you have to mine your own materials. Trading then becomes a much more important objective in a community so that some who like mining are useful and some who just like building can trade for different things.
I would be for having a vanilla server but again, I am just a new guy.