I was just asking for this in a different thread last week. As a player who joined the game after the Positivity Challenge, it's always bothered me that this is a part of the game that I haven't been able to experience. It's been years since the event ended, so people who participated at the time have had a lot of time to enjoy the exclusivity of the earned rewards. I therefore don't think it will harm anyone's perception of the game or of exclusivity if the challenge were unlocked again as something that could be played through. It seems terribly unfair that people who didn't discover the game at the time this event happened will never be able to play it.
The same rule should also apply to other timed events and challenges: leave them exclusive for a few years after they've happened, and then unlock them for everyone so that people who didn't know or hadn't started the game at the time will have a chance to enjoy them. The Sims team has made all of these wonderful events, and so, we should be able to keep enjoying the fruits of that work and creativity into the future. That would be a great way of adding some positivity into the game, and it shouldn't be too much of a challenge!
