I agree sorta.... but not entirely. If they were, say, 4 hour achievements then yeah I'd like to see people tackle them solo.
But the fact is 99% of the player base is not going to be able to achieve even remotely close to WR times solo. Considering WR times are in the 2-hour range, that extra hour isn't much of a cushion for the average player.
I'm not saying we need to make it baby mode so everyone can get them, but I feel like a 3-hour window and co-op being allowed leads to enough of a challenge that players without the requisite skill won't be able to obtain them no matter how hard they try, while others will be able to obtain them without hundreds of hours of practice, which is fair.
And in the days of Youtube, if anyone wants to tackle them solo for bragging rights, they can always broadcast to twitch with their XB1 with archiving enabled, then export the footage to youtube. No extra equipment required.
That and the in-game leaderboards will almost certainly have solo and co-op distinctions for speed runs, so we can all measure epeens until our eyes bleed, lol.