Okay, so you're saying the evolution is only meaningful if attached to a battle that hinges on league participation?
Does it matter if the very next episode were to hypothetically be Ash vs Tierno? No, will it have any meaningful impact? No. Will Ash have to re-challenge Tierno for a gym badge, no? Did Tierno make a life or death condition that if Ash loses the entire world will be destroyed, so Ash must win.....no.
You also mention it can be meaningful as a display of the strength of a bond, but that can occur in a variety of battle and non-battle situations.
Except it wouldn't, the bond between Pokemon and trainer only matters on the value and how vital a battle/situation truly is. Olympia was a gym leader, Ash needed a badge from Olympia (in this example) to move on with his own current goal of entering the Pokemon league.
True Ash could lose and rechallenge Olympia, but rematches tend to have less impact only in "certain" cases does the rematch fare better than the original match (if the original wasn't good to begin with).
Take this battle, any rematch would've been lesser quality. This battle was already top notch that it would've been an amazing loss had Frogadier and Talonflame not made it.
But the rematch wouldn't have been able to top it, it would have to be pretty much exactly the same but with a different outcome, and as a result NOT as meaningful.
Frankly if it evolves to save Ash's life for instance would be more meaningful than any gym match.
It wouldn't be as meaningful because they could've easily made it so Frogadier didn't have to evolve to save Ash. There's no instance I can think of where Frogadier needs to evolve to legitimately save Ash. Thus not meaningful. Now if we were talking about Ash's hypothetical Ivysaur evolving into Venusaur, and we know that Ivysaur can't save Ash on its own because Ash weighs more than Ivysaur can lift, then I can see your point.
Unfortunately does not apply to Frogadier/Greninja. At best, Frogadier would have to evolve to avoid "dying" kind of like Fletchinder into Talonflame.
But the issue comes from we've already seen, been done before "THIS" series. Thus loses meaning.