My thoughts on the subject:
There was a code posted by Sham Rock a few years ago that would go to the SSS instead of the Results Screen after a match. If you combined this with the vanilla feature of having the stage selection set to "RANDOM", the game would automatically transition from one match to the next, and change stage if applicable. The code seemed to work 100% fine on Dolphin, but it was prone to softlocking the system on console after a varying number of matches, but not too many. I never found a cause for the problem, and the problem only existing on console made it VERY hard to debug (all my development is done in Dolphin). At that point, I concluded going straight from a match to the SSS is unstable.
Now, that was a few years ago and we know a lot more now and could probably make more headway on figuring out what the exact problem was, but it is also quite time consuming to go back and forth from computer to console to test code with a bug that you can't reliably reproduce. When time is a premium, it is easy to tell yourself "okay, I think it's good now", when the bug might have manifested itself if you had just played a few more matches.
This is the main reason why Endless Friendlies in the 20XX Hack Pack goes back to the CSS for a single frame before transition to the SSS. But I do like the idea of binding a button/button combo on match end to enable Endless Friendlies for the next match (the auto-transition/stage select), ignoring the optional random character selection.
Btw, I have Endless Friendlies working again in the next 20XX better than ever, and with the option for four random characters per player instead of 3.