Did everyone just give up with the idea of having MimbleWimble as a sidechain? Was it a question of just taking the easier option and doing it as an altcoin?
Are there any mailing lists / forum posts with stuff about the old sidechain idea?
Is Andrew Poelstra still working on the peg magic?
Deploying a sidechain successfully and at scale is a full project of its own, and there are still some practical unanswered questions on how to do it. We just felt that focusing on one thing and try to do it well was best. Also remember that Grin had for goal to stay simple.
I believe Andrew has been busier with cryptographic constructs, like MuSig and Bulletproofs.
There is a good interview on Let’s Talk Bitcoin! #356 with YeastPlum which partially answers this question.
What I took away from that interview was that Grin is in some way a MimbleWimble proof of concept and one of the reasons for not implementing it as a sidechain is partly down to the fact that getting changes to Bitcoin is so difficult.
Why not both? 
Droolworthy: MW seems it would be able to transact in foreign tokens (“confidential assets”) almost as easily as the native token. The MW angle starts here Assets and Mimblewimble : Mailing list archive : mimblewimble team in Launchpad and I think the forum is lacking a thread on this topic that I’d be interested to help posting in (wink wink, nudge nudge)