Flush need to build a good player base first or a good community to have decent number of participants like what is happening in Stake's forum every week.
Stake has been here for a quite long time and has built a huge community. Flush needs time to build that community and to be honest, it's impossible to build the community with third-party slot games. They need to build their own games at the very first because all the slots are available everywhere in all the casinos. What makes a casino different is the in-house games. In-house games create the community along with some other benefits. I have been playing in a few casinos (though stake is the most) and mostly I'm trying their own games instead of playing other slot games. I enjoy playing Crash. Well, the Flush team said they are building the games but it will take time.
Basically it is possible to build a good community without in house games, but having in house games is absolutely an advantage.
I myself play slots from 3rd party a lot but I also play in house game especially plinko and keno.
While waiting them to rebuilding the in house game, they can start to build the community with the 3rd party games because I think building the community should be started as soon as possible without waiting for in house games.