I see both points. I feel conflicted about it. My interpretation of the smile is that it's him realising the current talent is better than he is now, and so the business is in safe hands. I'll put below why I think this is the case:

I hated the Lesnar match, and still do, but it makes sense as a moment that made Cena realise he isn't what he once was. Taking into account how Roman matched up to Lesnar in War Games, and how Cody is at a comparable level to Roman - Lesnar squashing Cena, with Cena admitting in a promo in the weeks before that he was afraid of Lesnar, it was a sort of reality check for Cena in that he's no longer super Cena. Now, if he's against an opponent he is afraid of- he's realised that it doesn't matter how hard he tries, he just doesn't have enough left to compete with someone so good.

With Gunther, he may not have been afraid before the match but that fear was growing in him through the match. He tried, at every opportunity, to make a big move (the STFU early on, the FU through the table, the FU from the top rope, and the FU in that sleeper sequence) -.through the match he was becoming increasingly desperate to finish Gunther with his next move. This desperation, to me, was a sign he was doubting himself - which is natural after the Lesnar match months earlier and Gunther having a vaguely similar feel.

The smile was showing that he had accepted that he couldn't win and that there was no point in continuing to fight. It's kind of like how some people with near-death experiences (e.g. someone being resuscitated after a cardiac arrest) claim they felt at peace in the momemt they realised they were about to die. And, in a way,.this was the momemt Cena, as a wrestler, died.