It is entirely possible that this will never happen in any reasonable time frame without re-engineering the whole system. If it can happen with relatively minor changes, it should have happened by now.
Speaking as a Subversion committer: Yes, you're right, it will still take a long time. It's very hard to make it work with a few small changes because the system contains quite a lot of layers of abstractions. We need to peel at each layer to make this work.
Each layer has a public API with some amount of compatibility guarantees. Which is both a blessing and a burden. It's a blessing for people who want to write tooling around Subversion, because they know that the tools they've written against Subversion at, say, version 1.0, will still work, without recompilation, with any subsequent 1.x release. This allowed a lot of third party tools of decent quality to be developed. No need for parsing command line output to interface with the version control tool (as was the case with CVS and AFAIK is still, today, with git). But it's a burden because it means we have to be careful not to break existing interfaces when making changes.
I wasn't around when the API compatibility guidelines were set up, and my life would be easier if they weren't there now. But the project is committed to keep them. Trying to fix things anyway is quite a challenge. It's very, very hard, and has to happen in lots of small steps, spread out over several release cycles. But it's a lot of fun, too.
We're currently rewriting the lowest layer on the client side, the working copy library. This will eventually allow us to do things like tracking local renames, so that tree conflicts involving a local rename will be solved more or less automatically. There will eventually also be improvements in other layers, e.g. client/server interface, and eventually the repository itself. Then we can start propagating rename information from the server to the client to close the loop and also handle non-local renames properly. When? Dunno. When it's done. It will take longer than many would like in any case.
If it is going to require major changes, somebody is essentially going to have to fork it and redo the core SCM storage from the ground up.
I don't see how forking would magically help with bringing about the desired changes any faster. You might just as well try to write a new and perfect centralized version control system from scratch. Or join the few people who are still actively committed to bringing Subversion forward and help us out. Subversion has already solved an awful lot of problems any centralized version control tool has to deal with. The glass is half-full when you look at it that way.
