My initial thought is SACD will not make a difference since these old recordings are of such poor quality anyway that SACD would just reveal more of the bad recordings
Many of those old recordings are great recordings, but I'm not convinced that the greatest of those by the artists you mention have made it to SACD.
I have around 900 SACDs. The only Ella on SACD I have (and I think is extant) is "Ella and Louis" on Verve - not her best work. And the one Sarah Vaughan I have is also on Verve and doesn't sound that great.
Still, there are plenty of other good SACDs - Bill Evans, Art Pepper, Louis Armstrong, Chet Baker, Ray Brown, Brubeck, Miles, Benny Carter ....