"While there are very real skills one can develop in a hyperbolic time chamber"
"we never want that to be an expectation added onto an already high barrier to entry."
First off, for those that don't know what the ROSAT is, please refer here to the above link.
Ok Rito, it's been pointed out several times that you're statement and reasoning in regards to no sandbox mode is contradictory, hypocritical, and downright makes no sense, so I won't go into that today. There was a Kotaku article published yesterday: http://kotaku.com/its-ridiculous-that-league-of-legends-isnt-getting-a-sa-1722535441
What I want to show you is that the example parody you give also works out against you.
Before the ROSAT training, Vegeta was not able to defeat Jinzouningen (Cyborg/Android) 18. And his arm was broken by her (infamous) kick.
Realizing that a Super Saiyajin is not invincible, he wanted to train. Led by Son Gokuu, he went in to the ROSAT with his son Trunks.
After coming out, he attained the 2nd Stage SSJ. This goes from him not being able to defeat 18 to easily able to defeat 2nd Stage Cell.
Noted that First Stage Cell (after absorbing humans) became quite noticeably stronger than both Jinzouningen 17 and 18. 2nd Stage Cell is a result of him absorbing the former. So there's a massive difference there, showing just how much the ROSAT training helped Vegeta.
Trunks too, he attained power almost equal (SSJ 3rd Stage) to 3rd Form Cell (Perfect or Kanzentai Cell, a result of absorbing both 17 and 18) and only lost the fight because his speed and dexterity were too low by comparison, raw power alone can't make up for that.
Pre-ROSAT, Trunks was only barely stronger than Future 17 or 18 (he could not destroy them as it'd always be a 2v1), who were weaker than the 17 and 18 in the main timeline, as noted by Trunks. When he went back to his own timeline, he very easily destroys both the Cyborgs with 2 moves, 1 each, and was even able to blast away First form Cell with an energy wave in his base form, easily clobbering and destroying him when he turned SSJ (the normal form).
Now comes Gokuu and Gohan:
Pre-Rosat, we don't really know how strong Gokuu was cuz he was in a heart coma for most of the saga, but during the training, he realizes the speed and dexterity weakness of the SSJ 2nd and 3rd stage and opt to master the SSJ form itself. An ingenius tactic, which ultimately proves fruitful as it allows Gokuu to fight at the level of 3rd Form Cell.
As for Gohan, when he entered the ROSAT, he could not transform, yet under the tutelage of his father, he attained the SSJ form, setting the record for the youngest SSJ of his time. And during the training he was able to briefly attain SSJ2 (though under the stress, he passed out and could not remember attaining the form, but his father saw it very clearly and based that to depend on Gohan to defeat Cell).
Ultimately, Gohan was the one to defeat Cell, and it was largely due to the ROSAT training.
Conclusion: Without the ROSAT, Cell would've likely ended the series.
So what's the point of this topic? Yeah, a sandbox mode won't physically make it so that 1 year in it equals 1 day in a normal game. But, their training in the ROSAT was not against their real opponent, whom were Cell and the Cyborgs. Yet they were still able to gain several levels than where they were at before they entered. Similarly, while a sandbox mode does not pit us against real players, if we train right (as proven by Future Trunks and Gokuu), we as players could attain massive results at much less time spent than if we were to play normals or bots.
So while it's not 1year=1day, it might as well be to an extent.
Thank you for reading.