By instituting the one-way transfer rule, they've permanently made it so that after a bunch of near misses, no one ever has to worry about No Guard Fissure Machamp.
To wit:
- It was possible to use TM27 to teach Fissure to Machamp (or Machop) in VC RBY, then transfer it into Bank, but doing so forced it to take the hidden ability Steadfast, rather than No Guard. It's widely believe that this possibility was the main factor driving the universal decision to give all VC transfers their HA if it existed at the time.
- The Ability Capsule which existed in Sun and Moon could not change that, only changing between the two non-hidden abilities and only for Pokemon that have two two distinct option for those (so Machamp with Guts or No Guard could swap back and forth between the two, but Steadfast was stuck disjoint from them).
- Once Home came around, Sword and Shield allowed Pokemon to keep their old movesets from Bank (unless you wanted to use them in ranked battles, which forced them to go through a sanitization process that replaced their old moves with level-up moves). At least the ranked ladders would get to stay clean from old-gen exclusive moves, while (unlike in the 3DS era) still allowing Pokemon from older games to have some pathway to participate in ranked, perhaps in search of the new Master Rank Ribbon that was added here.
- Sw/Sh DLC later added the Ability Patch, which at the time could only change a Pokemon to its HA, rather than from it, so your Machamp would still be stuck with Steadfast. But hey, at least least your other Machamp could get Steadfast too, not that they'd want it. (Also Grassy Surge Rillaboom and Libero Cinderace, yada yada yada, unimportant stuff like that.)
- BDSP and PLA later started the precedent that Home would track separate movesets for each game, and for all games except SwSh, entering a new game for the first time forcefully sanitized a Pokemon's moveset in that game, even if you didn't plan on playing ranked battles, ensuring that if your Machamp could ever get its ability changed, the collateral damage of its old moveset would be limited to Sw/Sh, not anywhere else.
- Scarlet and Violet changed the Ability Patch to be reversible. Machamp is not compatible with these games at all, even after DLC, but the precedent was set so that now it was simply waiting for the next time it got to appear in a game with reversible Ability Patches, and then it could change to No Guard and go back to Sw/Sh to at least make use of its moveset in those games.
- Finally, this announcement ensures that the next time Machamp does appear in a game, it will be after the generational boundary, and incapable of returning to Sw/Sh after all! Take THAT! Signed, Nishino.
Ultimately, though, Machamp is likely not the primary culprit behind the decision to institute a one-way gate. Rather, that seems to stem from the decision to add SP-LA as a language option, in addition to the old SP-EU (value 7). Whatever value the new language option will have, if Pokemon caught with it could transfer back to earlier games, those games would see the new value as corrupt data and not know how to handle it. Similarly, gen 7 games supported Chinese as a language option while gen 6 didn't, and all Pokemon (not just the Chinese ones) were barred from going backwards to avoid introducing compatibility errors.