Tell them it's free money, especially now that the value is low. If their post hits it mainstream they can really get a good RoI, but as with everything in this world, only the sun rises for nothing (if it's deciding to show itself).

Another really good argument I like is that it's still an upcoming projext and there are thousands of people waiting to be accepted (this is a fact), so if you're in early and manage to build up a bit of steem (literally and by figure of speach), you can really cash out once this hits mainstream. Remember crypto is still in it's infancy and I believe steemit will be one of the most successful platforms due to its shared 'wealth' concept for everyone (as long as you put in the effort).

I also struggle sometimes to find stuff to post about, for that I have 5 solutions:

1. Plan ahead, so you know what you're going to post about
2. Join @qurator, they have a channel that gives you inspiration
3. Join up with people who are likeminded, they will inspire you
4. Aim to do 1 post a day, if you have more inspiration, write it up and save it for a day when you need something to write about
5. Ask your 'audience' for inspiration, you get the best suggestions and posts that way

I hope this helps you argue your case. Feel free to ask more and hook up on discord.