My question is - What is preventing this? Is it just a matter of getting enough critical mass onto the new platform? Or is there some other logistical or engineering challenge that I'm not considering?
The way that Reddit is structured seems like it would be very vulnerable to a coordinated attempt to move users. In my experience, most subreddits have a heavily skewed distribution where a handful of users post most of the content, especially for the long-tail hobby and region-specific subreddits.
Intuitively, it feels like there are probably clusters of Reddit users who are subscribed to largely overlapping sets of subreddits, and who derive most of their utility from content generated by the same small-ish set of power users.
Depending on the terms of the investment, I wonder if some of these users might become very amenable to a migration to a new platform.
A quick aside. For those unfamiliar with Tencent, the Chinese government essentially has direct access to, and control over, all communication on Tencent's platforms. Nothing about the company in particular, its just part of doing business as a media company in China. But the fact of the matter is that you can be jailed for things you say in private group chats, and any chat can be arbitrarily censored, with no notification to the sender or recipient that the message was caught in a filter.