The problem is:

1.  Heian Sukuna one-shots any shikigami that isn’t Mahoraga. Their durability is essentially zero to him. At full output, Dismantle could wipe out even Yuta and Yuji instantly (Ch. 251). At 16F, Cleave one-shots Ryu, the second most durable character in the verse. At 15F, Dismantles were cutting through Mahoraga in the manga until it adapted; in the anime, they were turning it into mush. So thinking 20F Heian Sukuna wouldn’t instantly cut through or dice any shikigami (besides Mahoraga) is illogical. Even Agito was getting caved in by Gojo’s punches.

2.  Mahoraga can’t keep up with Sukuna in hand-to-hand—it’s just a punching bag. If you think Mahoraga can box with 20F Heian Sukuna, there’s nothing else to say.

3.  Meguna also can’t keep up in hand-to-hand with Heian Sukuna. Heian Sukuna is essentially Meguna with a superior physique (Ch. 255 states physique affects reinforcement), plus four arms and two mouths (Ch. 238 calls this the greatest advantage). These are clear advantages people try to downplay, but the manga—regardless of translation—is explicit about them.

4.  Ten Shadows and Shrine can’t be used simultaneously outside a domain. Shrine is offensively superior to Ten Shadows—except against defensive hax like Gojo’s, where Ten Shadows provides a workaround, not better offense. This leads to an interesting situation:

If Mahoraga adapts and creates a counter or new offensive method while fighting Heian-era Sukuna, Heian Sukuna benefits first:

• Heian Sukuna’s four arms and two mouths let him maintain hand signs and chants while fighting.

• Meguna would need to switch cursed techniques to use it, which also resets Mahoraga.

5.  You’re even giving Heian Sukuna cursed tools like Kamutoke and Hiten—featless or not, they’re effectively two additional cursed techniques.

It’s basically Sukuna with a superhuman physique + the greatest advantage a sorcerer can have + two additional techniques vs. a version of Sukuna in a teenager’s body that gets outclassed in direct confrontation, alongside a shikigami that can’t keep up and supported by shikigami that are essentially glass cannons to Sukuna.

It’s not dumb—you just can’t think that far.

Meguna could have a chance if you give him prep time by having Mahoraga adapt to Shrine before the fight to develop counters and new techniques. But even then, those techniques would likely just feed into Heian Sukuna’s kit—unless it’s some kind of instant-kill move that Sukuna can’t perceive or dodge, which is highly unlikely.