It depends on how big the pool is. Minecraft is working with an ocean of people while Landmark is currently a kid's pool. I wish there was a magic way to see the percentage of people who bought it and actually created something worthwhile versus those who got sick of stacking blocks after a week. I don't think 20 percent is that crazy of an assumption.
I think that number would mean a shitload of content if you've got millions of players. Landmark doesn't, nor will it reach that 20 percent mark if it opens to anyone other than the people that paid to alpha test it. Aside from my initial statement, I really don't think we disagree (that much?). Landmark's tool set is deeper and far more complicated than Minecraft and the tools don't compensate that curve. They're a mess and I got frustrated just properly copying and pasting shit. I damn sure won't be the go researching hours of Youtube videos on how to make a curve. That should be built into their system (I'm sure it will be eventually).
Notch checked all the boxes for mass success and head the right amount of luck. It runs on fucking everything, it's got an aesthetic that absolutely works for its cause and it's almost as easy to work with as stacking Lego's together (actually probably easier, forgive me, I was restricted to Lincoln Logs).