WhatsApp is fundamentally different from Reddit. WA matters if the people you want are on that platform or a different one. Reddit has a network-effect but you care less about specific people and more about content/discussion. If my brother in law is on WA then I might have to be on WA, Reddit is much more fluid. If somewhere pops up with enough content/participation then I'm good to move.
The other major difference is content. On WA you only care about the content your direct friends/family create, on Reddit the masses are dependent on a few to submit comment (and even comment, there are a ton of lurkers who never comment, might not even up/down vote). If WA content goes to shit you can tell your BIL to stop sending you crap, if Reddit content goes to shit (due to loss of power users or moderation) it's a much harder problem to solve.
The WA exodus always struck me as way too difficult, getting friends/family to switch messaging platforms is super hard. Reddit? Might be unlikely but I was in the Digg->Reddit migration and I'm 100% looking for where I want to move now that Reddit is acting this way.