Hymn attacks a resource that doesn't matter unless you're winning/poised to be winning. The moment they reach two mana and play any onboard value engine you are now losing. Hymn might be a "good" card b/c it's a 2-for-1, but they're drawing 2 cards a turn and you're not really addressing that issue by taking a turn off to Hymn. Trophy has multiple decision trees you can follow, one of which includes killing their turn 2 value engine such that you can do things like play a Delver again (looks pretty bad when opponent has a Wrenn, you Hymn them, and yeah...you still can't deploy a Delver while each turn Wasteland and Daze keep accruing irrelevance).

This is one of those points where you need to look at your deck's composition and apply the theoretical scenario of "my opponent has a Wrenn." Of your 60 card deck 18 lands + 4 Delver + 4 Daze + 4 FoW + # of Drown/Hymn = 33'ish cards that can't really fix the problem. Then you add 8-10 cantrips and 4 Wraith, which will only be used to find Gurmag or Shadow. There's not any left over space in your deck to do things differently - so you now have this deck that can only be played one way; the result is that all your individual player skill doesn't really matter. You're simply managing a deck, and the opponent has the luxury of now playing against predictable forced moves. All your moves are pretty slow as well (sorcery speed and summoning sick); your deck is lacking tools to incite an opponent to enter your turn with any tapped lands.
If my pile opponent plays a wrenn and ticks up and then I trophy it and they find a basic I've traded 2 mana for 2 mana and I am down 2 cards. Not interested. If my opponent is on RUG they can't find a basic but they are still up a card and they have Daze/Snare to fight over Trophy. So if my opponent resolves Wrenn against my Hymn deck then sure, I might be behind. The same is true if they resolve Jace against my UB creature deck (or Moon, or Chalice, or B2B, or whatever). Rather than panic and say "oh shit my deck must include clean answers to this resolved permanent" I am hedging towards making it difficult to resolve the Wrenn in the first place (with my 4x TS 4x Daze 4x FoW 4x Hymn deck). Maybe this approach is still not a winning strategy against the Wrenn decks, but I can't help but feel that if that is true then the solution must be "play a wrenn deck" not "play a bug deck with assassins trophy in it"

Now Hymn can randomly win a game all by itself, it's just not reliably converting and we know that b/c the Hymn spammers are nowhere to be found.
I wrote why I think Hymn is bad in Pile decks atm but could be good in aggressive strategies and you still haven't addressed it and just keep repeating this same vapid point over and over

We're at a point in legacy where you need to attack a different resource with multiple card types, rather than wholesale hand destruction. When it comes to other Delver decks every single successful one is playing Wrenn, or Oko (which can kill a Wrenn), or Dreadhorde (the pseudo-PW) - so yes, I am saying that is the case. Delver + Hymn is a losing proposition, Delver + any [or all] of those is how you win.
Your explanation for "why" here is meaningless ("we're at a point in legacy where you need to attack a different resource with multiple card types") so if you peel that away then what you're saying is "how you win is by playing these decklists that are currently winning" which is hardly a masterstroke of analysis.

In terms of the UW example with Trophy, just let them begin the game with JTMS in play (turn 0) with this condition: you can't use white mana - this is a pretty easy win for Shadow. People that play UW understand that [if they have the tools] they have to fight over anything that threatens their access to white mana. Few decks outside of R/G Lands can really call them out on that weak point, but do understand that this is the weakest point of UW. Any time you make the white side of their deck count for nothing, they lose convincingly.
Turn 0 JTMS with only access to blue mana might be losing but that's not a realistic analogue. If you're hitting 2 of their basic plains with trophy DURING A GAME then it means they must have had access to white mana at some point. They can make sure they don't get their W sources deleted for free by only fetching when they have a W spell ready to go. Trying to thread this needle with a counterable LD spell from a BUG delver/midrange deck is going to be pretty fucking hard. It's not like lands where you just recur Ghost Quarter for free over and over.

It's important to step back after reading what a card does and figure out what it really means. In the case of Trophy you're not just backing up a train of dudes, you're looking to create a subgame your opponent has to sink resources into, such that they can't sit there focusing solely on killing your one-dimensional combat step wincon. You can't create that subgame by throwing raw power [Hymn] at them b/c it's already outclassed by their raw power [Wrenn/Oko mostly]. It's definitely easier to give up and just play Wrenn/Oko/Dreadhorde + Delver, but you can still play dudes-only if your spells are creating a virtual not-creature threat, while also having dual utility to support your dudes.
Again, this is just a word salad, watch:
In the case of Hymn you're not just backing up a train of dudes, you're looking to create a subgame your opponent has to sink resources into (either they get 2-for-1ed by Hymn or they have to spend resources fighting over it), such that they can't sit there focusing solely on killing your one-dimensional combat step wincon. You can't create that subgame by throwing removal [trophy] at them b/c it's only a 1-for-1 (or a 1-for-2 if the opp has basics) and you're trying to hit things that already provided value on etb [Wrenn/Oko mostly].
This makes much more sense than shit like 'your power is outclassed by their power' as if it's some kind of DBZ episode

Lim-Dul's Vault
In a non-combo tempo deck there's zero chance I play a 2 cmc spell that's card disadvantage.
I think Plunge into Darkness is probably better and I wouldn't play that either