The advantages provided by those are neither lasting nor exclusive. The rewards you could obtain from the Challenge Rift are also obtainable from bounties (and to a greater amount as well); as such the advantage is mostly a short term advantage, as within about a week of playtime (assuming you haven’t done any bounties), you’d already had burnt through the content of the Challenge Rift reward from activities such as cubing legendary powers, reforging legendary items, converting a rare into a legendary, utilizing the mystic, etc.
Powerleveling advantages vary and in some circumstances possess some tradeoffs. For example, if you get powerleveled early in the season, you’re least likely to have any decent starting gear, as such you’ll then find yourself looking for someone who’s willing to carry you in a number of rifts and/or greater rifts. Trying to find someone like that (even with the communities function), you’ll be using up time that you could be using for activities such as bounties. The only time I would say that powerleveling is truly advantageous is either if you already have a bunch of exp gear ready and plenty of gems of ease to use (most of which would’ve required some time and effort to use) and/or you’re being power leveled by either a friend or a kind soul who also happens to play a class similar to yours and such the gear they drop to you has use besides being cubed or salvaged for souls.
Now regarding solo players, they have access to the same content as multiplayers do; as such the disadvantage they have isn’t in terms of exclusivity, but efficiency.
In short, even if I were to miss the first week’s challenge rift, I can still get bloodshards, crafting mats, bounty mats, and gold from other activities. Even if a friend of mine gets powerleveled and plays in a group with others, while I continue to solo on; I’ll still have access to the same type of rewards as they do, he’ll only get them at a faster rate.
On the other hand, if you tie an exclusive item with endgame value in into a temporary event like Darkening of Tristram, with no other way of getting it; then for any player who missed out on it, not only would they not be able to catch up to the players who did partake, but for those who don’t care about competition, they would lose out on a potential key piece of item or gear that could be used for making potentially new and/or interesting builds to them.
For example if I were to run the Darkening of Tristram event (which I always run come every January) and completed it while obtaining the item, but a Friend of mine wasn’t able to for whatever reason, then he would’ve missed out on a reward that no amount of time or effort can help him get; and by the time January rolls around again, the season would’ve been over.
So yeah, I rather not have time-based exclusivity, unless said exclusive item is purely cosmetic with no impact to a person’s gameplay; otherwise there would need to be additional ways for players to obtain said rewards.