there's one digimon your forgetting about & is the main reason why ken went all dark emperor to begin with:
millennniumon. it has the power of both time & space. not only that but it's was also the incarnation of "even if you kill me, you won't stop me" & "this isn't even my final form"
even if you kill millennniumon, its consciousness moon=millennniumon will separate from its dead physical body & the physical body will be revived in space-time.
by the time it becomes zeedmillennniumon(via moon=millennniumon possessing its own corpse) it becomes capable of cosigning an opponent to oblivion in time & space.
dialga & palkia are fully capable of getting killed and its likely there's a more then a few points in time & space where they can get outright killed by something or another. its also doubtful they'd be capable of being revived on their own.
your also not factoring in the difference in the rules of physics between each digital world & the real world it's connected to.
let's look at some of these differences:
in the adventure world, digimon can die in the real world but can also control their data to some extent(wizardmon went a few years in various electronics & miyotismon possessed oikawa)
not only this, but daemon was fully capable of opening digital gates on his own. odds are millennniumon could have done the same.
in the tamers world, digmon who died became data capable of being seen, with no indication of them ever coming back outside of leomons spirit manifesting itself when beelzemon used fist of the beast king.
one of the main differences is how digimon manifested in the real world(s). yes, theres was that electronics going on the fritz that seems to happen in all of them to some extent, but tamers had something different: bio-emerging. adventure, frontier, data squad, fusion.. none of these worlds had this. there's also a difference in limitations.
in adventures, tamers data squad, & fusion digimon could get into their real world with no real problems once they got there. with frontier on the other hand, normal digimon simply couldn't get into the real world. the only digimon that managed to do so without causing a massive distortion was technicality a human.theres also more requirements for digimon to get into the real world in fusion then there are in the other ones where its possible. adventures tamers & data squad just need a small distortion.
now if we are going by the physics of the pokemon world, honestly it would be a lot easier for a digimon to get into the pokemon world from some computer. how can we know this to be true? simple, we look at the pokemon that closet to a digimon in nature:
that right, the porygon family all known as virtual pokemon.
they are data.
basically, any digimon that would come from a digital world running parallel to a pokemon world would have an easier time getting in & and out of pcs & such. this means that everything could easily get loose.
even if its a porygon only trait, it would be easy for various digimon to either copy the power or completely hi-jack every porygon that ends up in the digital plain & making them transportation.