To focus a little more on the non-sense of the timing of allocation of teams into the group stages for the 2019 RWC (and the even more ridiculous timing for the 2015 RWC), you need to look at where teams are in their developmental cycle.
There is a four year cycle leading up to the only proper global tournament in rugby - the RWC and all teams who want to win that tournament go through a four year developmental cycle with the objective of being at the peak of form and development at exactly the point when the RWC takes place.
So we are currently 1 year on from the last tournament so not surprisingly most teams are at an early stage in that cycle - teams are in transition, dropping players who are unlikely to be competitive in 2019, perhaps because they are too old, trying new blood and new tactics to see what is working. Some teams (e.g. England) seem currently to be being pretty successful, while others (look at the selection of SA last weekend) starting with a whole raft of inexperienced players.
Now that isn't really a problem for SA, or at least shouldn't be if the tournament bosses were competent - as it matters not a jot whether SA get spanked in a friendly in Cardiff in November 2016, if they build toward having a formidable team come 2019, which I imagine will be the case, as will be the case for the other SH sides.
But we run the risk that if SA drop a touch down the rankings now, as they are in full on development mode, that they might end up in 2019 as a top ranked side, but end up n a group with other top ranked sides, as happened in 2015. It is non-sense, it was non-sense in the run up to 2015 and the organisers were massively criticised for it - yet they are doing the same thing again.