No, it’s not worth it just for the guaranteed seat would be my answer. Those “guaranteed seats” has to be on VA operated flights only, which is very limited routes and as John suggested, it doesn’t fly to JNB so you won’t get those seats.
Also those “guaranteed seats” are in economy class only. All of us in the community all agree that redeeming points in economy class is not really worth the money especially VA usually come up with very competitive sale price on their economy class seats that makes redemption even less worthwhile.
There are other great perks for being a velocity platinum members that IMHO, if I’m only a few more SC from getting it, I will go for it. Not for the guaranteed seat, but for those other perks.