I don't think it's simplistic to view a game being all about the players.
But it is simplistic to view the game world (or its associated mechanics) as ending at the limit of what the PCs can see, which some here seem to be advocating.
I don't think you have to throw immersion out the window to make the game actually play like a game, but I think some allowances are to be expected. I think labeling that awareness as "simplistic" is a bit harsh.
Perhaps. But immersion comes from believability which comes from realism where such is possible; and from consistency. The game mechanics in effect become part of the game-world realism and thus must be consistent...and that mechanical consistency then needs to extend beyond what the PCs are doing at the moment.
Which means that while the DM is free to simply narrate remote events (100 knights went up the pass, only 17 came back) rather than roll all the dice, that narration has to reflect something that the mechanics could reasonably produce had they in fact been used. (unknown to the locals as of yet, several dozen stone giants have just tunneled to the surface up there, along with assorted pets, and are claiming the territory for their own)
That's like saying that if tweaking the encounter guidelines in the way described in the area in question causes the illusion of the fictional world to break, then the illusion probably wasn't all that strong to begin with.
That's just it - the illusion is only ever as strong as the DM and players make it, thus if the game system is forcing the DM to weaken it that's not helping.
I think the missing pieces are the frequency with which this change is made, the area to which it's applied, and the amount of time the PCs spend there. If you have an area that's been established as relatively safe, and the PCs travel there once and come across three deadly encounters....that can be attributed to bad luck or bad timing or just random chance. However, if this is an area that the PCs will visit regularly, and all of a sudden there are epic level threats crawling all over the place....sure, that's an impact. Seems odd to go that route, and I'd say maybe there's a better way to handle it....seems like an active attempt to break immersion, if you ask me.....but if that's the case, then sure, thought needs to go into this and how to handle it.
The problem isn't "safe" areas, except as you say if said safe areas aren't really safe at all. The problem is "risky" areas (the road to Waterdeep being a fine example), where adventurers face threat and commoners by extension thus face certain death; meaning the encounter guidelines as written almost force a points-of-light type setting and thus absolutely affect worldbuilding in that they are dictating what type of world you can (wth any expectation of realism) build.
Now if there was a great deal less difference in abilities between commoners and mid-level adventurers this wouldn't be an issue at all. But the greater that difference becomes, the more care has to be taken as to how all these threats to adventurers are going to affect the common world - if one cares about consistency, which I do.
But most of the time, I expect that the amount of time devoted to an area with a few random encounters would be pretty small as to not disrupt the campaign setting. The impact would be negligible to the point that addressing it would be more disruptive than to simply let it go.
If it's only one area and it's only met once (or it's only deadly once, for whatever reason), then fine.
But if it's a lot of areas (either by actual PC interaction or by tales and reputation), then simple extrapolation is going to very strongly suggest that those areas are now becoming representative of the game world as a whole, and bang goes your worldbuilding again.
Lan-"and if elite knights are dropping like flies, maybe the 3rd-level adventurers might want to look elsewhere for their fun"-efan