It doesn't have to be millions. What if profit is not a motivation? What if you just want to be the host of some good communities or a less toxic web2.0 experience?
r/Compilers has about 14,000 members. Again, how will you find that many people willing to contribute on that topic to make it worthwhile?
r/ProgrammingLanguages has about 90,000 members.
You have a chicken and egg problem -- until you have enough actively participating members, there is no benefit to moving.