The vast majority of those sales are for GTA online, which this leak doesn't inherently give you access to. I don't see this leak financially harming Rockstar more than the cost of the presumed ransom, people still have to pay to play GTA online.

Give it time and there will be private servers for the modded client

There are already private servers. There are whole communities built up around role playing in bootleg GTA 5 servers.

For context, one of these is FiveM[0], which is very popular (and if anything, a lot better than the official Online in many ways.)

[0]: https://fivem.net/

Rockstar bought the developers of the private server software out and integrated them into their teams, presumably for GTA 6.