Unless I'm missing something, I'm not seeing a problem here provided the OP turns OFF the computer before putting it in the bag (as opposed to just sleeping it). There's nothing that should move and the computer is off, a little padding to lessen the shock is about all I'd suggest.
I'm not trying to be a wise guy here...so please don't think of what I'm going to say in that way.
But when was the last time you rode a bicycle? I only ask this because I know that probably 95% of adults probably haven't ridden a bicycle in the last 5-10 years. And even if someone does ride a bicycle regularly...they may not be very familiar with how bumpy a ride it can be...especially when items are carried in a bicycle basket or bicycle bag (panniers).
Occasionally I'll ride my mountain bike to the grocery store. I have two types of carriers...the rigid metal cage type...and the soft-sided folding canvas type. If I'm using the rigid metal type...and I have a bunch of metal canned items inside the plastic grocery bag (in the metal bicyce basket)...you would not believe how much those metal cans get jumbled & jossled around. They are constantly banging, clanking, and vibrating up & own with every little small bump in the road surface. And if a larger bump is encoutered...BAMM...some of those canned items practically jump out of the basket. Same thing can happen with a 1 gallon jug of milk.
The road surfaces in my area range from super smooth...to mildly bumpy (semi-worn road surface)...to REAL bumpy (road needs to be resurfaced ASAP)! Folks really don't realize how bumpy road surfaces can be when they are riding in their automobiles...with fancy-cushy suspensions..and big air-filled tires. The bumpiness of roads can really be experienced when riding a bicycle...and much much more...when carrying items in a bicycle bag/basket.
Given this...I would certainly not want to expose an expensive laptop computer to the same vibrations mile after mile, day after day...on a daily commute without some serious padding & vibration/bump protection.
Yes...a MacBook Air does not have much in the way of moving parts that could be damaged by vibrations (initially). Where the damage occurs is...the constant bump bump bump bump bump vibrations the laptop experiences every second/ every minute/every mile on a bicycle. If this happens enough times (long term)...every joint, every mechanical connection, every surface that rubs against another surface will slowly but surely wear & loosen up (laptop display hinges for example).
Ever hear the difference between the sound that is made when a brand new automobile door is closed...and a 10-12 year-old automobile door is closed (creak creak...squeak squeak...clang clang...clunk clunk)?

This is the same sort of thing that can potentially happen to a laptop exposed to too much/too many vibrations (without proper protection).
- Nick