It's important to realize that when a deck is just 1-card combos
I have to stop you there because this idea has no clear definition. I assume you mean just like, good cards?

that there's nothing going on in the background that guarantees continued relevance. When playing a deck like that and power creep happens, the deck ends up falling into irrelevance [tier 3].
Ok but there's nothing to stop powercreep also affecting "non-1-card combo" decks like SNT or ANT either so this assertion seems meaningless

To regain relevance [tier 2], the answer will never be doing the underpowered thing harder/more consistently. This is best exemplified by decks like Mentor and Blade, who keep doing this consistency stuff but never fixed the part where they're underpowered...so they're consistently underpowered [tier 2.5].
"If X deck is not good anymore then it needs to change somehow if it wants to be good again" Sure, isn't this blatantly obvious

Now Shadow has things going on in the background which it can fall back on [the life loss engine]
What does "the life loss engine" even mean. The only card that rewards you for losing life is Shadow itself and it doesn't do anything except be an efficient threat

but everything on top of that is a field of 1-card combos which are all being power creep'd out. The solution isn't doing the thing harder, you have underpower problems.
Sure, this is why I am experimenting with greater numbers of midrange/value/control cards like Hymn/Strix rather than just being Grixis Delver sans red

Ponder and Street Wraith are not adding power, and to address a power problem you will need these slots. Otherwise you're going to be waiting a long time for strictly better 1-card combo threats [creatures or undercosted PW] to get printed.
Absolutely not true that these are the slots (ponder/wraith) that are the most inherently 'swappable'

The worst thing you could do with Shadow right now is to add even more consistency slots [Baubles] while you have an underpowered problem, which also happens to be running directly into Prismatic Ending and aggressive Bolt-using decks. Consistency is meaningless if the payoff is underpowered, and poorly positioned.
Bauble is not very good as a 'consistency' card because it just replaces itself with no selection and a 1 turn delay. It's only good if e.g.
a) you care about artifact in the graveyard (e.g. Delirium)
b) you care about knowing the top card of your library (e.g. Predict / CB)
c) You care about casting a ton of cheap spells (Mentor or something)
d) Auriok Salvagers or Mox Opal or whatever other synergy
The only suggestion of bauble was specifically as a DRC enabler, not because the deck vaguely needs more cantrips or something
"Cut this spell and add a cantrip" isn't necessarily good or bad advice, it depends on the context

Shadow needs to change, or it's only good matchups will be vs combo.
Ok, I dont think anybody really disagrees

You know the wincon is picking fights over the mana since Bant midrange is tinkering with dangerously low land counts and the Delver-types also have suspect mana. These are your dominant problems [Ending and Bolt], and whatever is happening with the Saga matchup is also going to be about mana. Green plays nicer with heavy UB openers as it comes online later with cards like Trophy [the mana attack]
Using trophy as manadenial simply does not work against decks with basics and this includes both Delver (UR being the most popular variant) and Bant

and Library
Library is a good card but infernal contract also usually does the job in matchups where you want this effect without adding the extra colour

and Berserk [remember it kills Ragavan]
Berserk is exactly the kind of effect that "does the thing" (combat step) with more dedication, I thought this was exactly the kind of effect you were trying to avoid? Using it as removal for Ragavan is also pretty awful

you also have an option to absorb a non-Pyroblast'able PW in Grist (which is better positioned than Last Hope)
They do different roles in different matchups.
The huge problem with Grist in this deck is anytime you cast it there's 0 chance you have any creature you're happy to sac.
You either sac one of your good creatures to kill something and hope they don't have a follow up to kill Grist or you just cast it and plus to make 1 insect and again hope there's no strong follow up, neither of these are very appealing. (Of course if you are way behind on the board against big creatures then Last Hope is also not great but at least Liliana is very good at being a kind of Night of Souls Betrayal

and it's a pretty good time for Deed's asymmetrical attack vs Urza's Saga
This is kind of interesting actually but maybe in a different deck

You know what needs to happen - you need those slots Wraith is stealing...because you need power, before consistency.
By this logic every blue deck should just cut its cantrips for "powerful" cards. ("I can't decide whether to play Endurance or Icefang. I guess I just cut my cantrips and play both"). This is just lazy/wrong deckbuilding.
Wraith almost certainly shouldn't be cut because it's too powerful of a Shadow enabler. The question is which "powerful" cards to play in the remaining slots