The press releases for the first two Sony Japan Santana releases were chock full o' verbiage about how they were commemorating the 50th anniversary of the original release, etc. so I entirely think that's the motivation here.
The 1973 thought is a nice one, except that Sony Japan only seem to do one artist at a time when it comes to these quad SACD releases - on the pop side it was Jeff Beck, then the aborted Simon & Garfunkel disc, and now Santana. On the jazz side it was the two Miles Davis discs, and then Herbie Hancock's Headhunters, so you'd expect (hope?) to maybe get Thrust this year in addition to Santana III. Who knows, maybe they'll suprise us with more, or something else entirely, but good thing there's another label delivering quad SACDs in superior sonics, isn't it?