Zero impact on Reddit.
Reddit has 52 million daily active users and approximately 430 million users who use it once a month. In comparison, Lemmy has a mere 143k users and kbin has about 33k users. Reddit hasn't noticed that anyone left, and it never will.
Reddit will consolidate its platform and fully implement its ad business. 99.9% of users will stick around because they don't care that much about 3rd party apps. Reddit will then have a big revenue stream from ads, allowing it to achieve profitability and do an IPO, which will bring in another boatload of money. At that point Reddit has plenty of cash to invest in improving its in-house app, mod tools, spam filters, etc.