You either don't get it or don't appreciate it.
I just reject the premises of your argument. To recap: This rule breaks Sorcerers if they can meet both of the following criteria:
1. Perfect knowledge of what the next encounter will be.
AND
2. Effectively unlimited number of extended rests with which to sculpt their spell list.
Let's start with the second premise. The fifth level example you throw around requires 6 days, so about a week with zero encounters in the interim. Apparently, we've gone past the 15-minute work day, and are now into a 15 minute work week!
(This is also why I haven't bothered to discuss your proposed downtime house rule, if your tables are under such little time pressure that taking a week off before the next adventure is trivial, then why not just make it 6 weeks?)
As for the first premise, that has been pretty thoroughly gone through in this whole thread. Sorcerers lack most of the scrying and divination spells that would give the player in game knowledge of the spells they need, and as much as it'd be really cool, I am not currently gaming with Paul Atreides and Cassandra.
So, no, I don't see this as a broken rule, I see two primary use cases:
1. I've made a huge mistake, ie, the player has picked a spell that does not work the way they thought. Key example, the number of players that continue to pick
Crown of Madness and keep casting it on the solo monster.
2. Adapting to the environment. Sure, if the campaign has lead the party out of the wilds and will be in a city for the next few days, the Sorcerer might say, "Well, not gonna need Alter Self for awhile, but Enhance Ability would be more helpful here".
(A third niche case as well, is for Wild Sorcerers in particular. I've had players discuss building a "random slot" where one of the Wild Sorcerer's spells is assumed to be retrained after every rest and they roll randomly on a table at the beginning of the day to find out what spell they actually have that day.)
These do not strike me as game breaking changes. They give the Sorcerer an advantage, for sure, but then, this is the same rule being given to all casters with a "Spells Known" column. Rangers, Warlocks and Bards are also getting this boon, and I don't see the same level of "Oh no, the sky is falling" with regards to them. Prepared casters have been customizing their spell selection since day 1 and it hasn't broken the game either.