Like if Dreadnought was a good card by itself as a big threat in a deck with stifle then why would it not be successful in a more generic kind of delver/tempo deck without standstill?
If standstill was a good card by itself in a kind of stifle/tempo deck (with e.g. Factory, presumably, but not necessarily), why do we not see people having success with this kind of strategy without dreadnought?
UR Standstill/Landstill can't beat combo [particularly GY-based] to save its life. It also really struggles to count to 20; which is a problem when you run Bolt. So while you could play some kind of UR Delver w/ Standstill, you have some significant problems with Chalice, and just like UR Delver the deck is completely unplayable vs DRS/Oko/Uro/Klothys/Cling-levels of lifegain. Standstill might feel cool in such a list, but Delver and Daze don't exactly play nicely with 7 enchantments (Standstill + Shark'nado), and you can't slow down b/c you're on the crappiest color combo vs Goyf. You just can't afford to play a card that will extend a game [Standstill] when you don't have a solution to the Goyf problem, and are sitting on Bolts you can't reliably pressure with...and also sit there drawing dead soft permission [Daze]...and also have >20 lands to further boost dead draws. This gameplan has too many holes to overcome. This Blue Moon-style of deck just isn't competitive - just play UR Delver or SnS or Dreadstill and take the higher win %.

On the flip side [Delver + Dreadnought, no Standstill], you don't get to pick a spot; if Nought is in your hand and you can make it, you have to - you don't have a late game. If you want to play linear without any comeback mechanism or inevitability, play DS and stop 2-for-1'ing yourself. Also accept DS's complete inability to handle Chalice, and like I dunno...tell yourself you have game vs it b/c of Brazen Borrower. Also accept your unavoidable losses to Vial, Plow, Counterbalance, and Delver decks with Bolt.

Both paths have either no meaningful asymmetry [StifleNought] or no effective asymmetry [UR Standstill/Landstill]. This is something of a problem since effective asymmetry is the wincon. If you had to choose one of those, you should recognize that Scroll of Fate by itself is more likely to win through dubious construction. In Dreadstill we have the tools to race combo and still have late game inevitability. If you only have one tempo reservoir [Dreadnought or Standstill], you have locked yourself into one speed of play. No matter how skilled a pilot is, they can't turn skill into win % outside of that speed the deck is locked into.

What is the perfect confluence of factors that makes these cards necessarily pair with each other, in 2021?
The difference is Standstill and Dreadstill have a strategy - most legacy decks don't. One way to tell if your deck doesn't really have a strategy is if a 1-card combo [power creep] invalidates your deck. If cards like DTT, DRS, Breach, Lurrus, DHA, Plague Engi, DRS, Shepherd, Muxus, and Uro are sending you to frown-town, it generally means you're playing mono 1-card combos yourself...so when they print a better one card combo, there's nothing really hiding behind your pile that insures continued relevance.

So unlike your typical legacy deck, Standstill and Dreadstill are much less affected by change. Sometimes cards are ridiculous enough to affect our color selections or force us towards Dreadstill or Standstill/Landstill [ex. Oko, Wrenn], but these cards don't send our winrate below losses, and these cards get themselves banned. This is to be expected from decks that existed in entirely old-border and have maintained relevance while doing basically the same things as always [Standstill/Landstill, Dreadstill, Goblins, Elves].

At this point the most realistic explanation is that those cards were simply the best options available at the time in terms of cheap threats and cheap card draw (this is like pre-delver, mind, years and years and years ago) and a post-hoc rationalization was made to justify that the deck was built with some kind of special synergy.
So these decks average 70-75% winrates in competent hands. That's why we play them. We also get more tools more frequently, and that makes us the best Teferi [3 and/or 5cmc], TNN, Azcanta, manland, and/or Shark'nado users in the format, depending on our build.

Why not try expressive iteration or etc etc
Card is trash. Here today, gone tomorrow...like AK, Predict, Night's Whisper, Ancestral Visions, JTMS, Painful Truths. If I'm spending 2+ mana to cantrip, I'm going to enforce asymmetry [Standstill] or have lategame [Shark'nado, Azcanta] or react/control [SCM]; I'm going to get on-board and push a gameplan/enact a strategy. Blind value obviously works in magic, I just don't find it interesting or skill-intensive. I also don't like the tendency of blind value strats find themselves spinning their wheels trying to draw effects they don't have access to because they failed to enact a strategy b/c they wanted tap out and jammy jam Uro-types.

But ultimately I think Dreadnought is just not a playable card (it's basically blue pox in the sense that people get a nostalgia buzz out of doing something old/cool that mostly sucks)
People can keep thinking that all they want b/c they still lose to Dreadstill and Standstill/Landstill - especially if they're on tier 2 (that's like 85% or higher losing to Dreadstill). The main thing is that decks with Standstill lack dedicated pilots. Every so often we have more tourists than usual, but very few of them stick with the deck. They aren't leaving b/c they're bleeding money.

Your assessment of the deck as "worse shadow" is basically in line with my assessment, I don't even think the Shadow matchup vs UR is bad
When it comes to StifleNought, yes. A pilot is better off playing Infect or Shadow; these are the exact same thing as StifleNought with less 2-for-1s, and a slightly different weak-to-X profile. I would however never want to be playing Infect or Shadow or StifleNought vs Dreadstill; this is a mirror these decks didn't come ready to play.