Honestly, they do that well enough as it is in some cases. I loved how you just moved on in the story if you were defeated at certain points - my first time through I remember losing to Sina, Hau (on Ula'ula), and Lusamine (at the Aether Paradise) and instead of blacking out to challenge them again you just kept going in the story. It makes the loss sink in a little more, provides a little frustration with the fact that you can't try again.
I wouldn't mind if Gladion had been so overpowered when you first meet him that you lose no matter what - he seemed to have the right kind of character and personality to pull that off nicely.
Honestly, this is what I was expecting to happen before the game's came out.
I really got the vibes from Gladion that he'd be an unbeatable boss the first time round, with maybe one or two Pokemon that are tough but beatable, then a much higher level Type:Null sweeping our teams to establish Null & Gladion as a serious threat.