Will the two elastics cross sections stretch loose over time?

All elastics will lose some stretch over time, even the best of quality. With that said: 

One, the Original Manta SLEEP Mask uses an extra wide and triple enforced elastics band that is going to last significantly longer than anything you’ve probably ever used in a sleep mask. These are industrial grade elastics. 

Two, the product only has 2 small cross-sections made with elastics, rather than the entire strap. This way, even if the elastics get slightly loose over time, you still can adjust the size using the micro hook and loop strap closure.