I found the content of your post to be a deliberate exercise in comparing apples to oranges.
Comparing combat replacements into veteran units seems pretty darn apples to apples to me.
Ideally, of course, in the real world, you would only pick veterans who are equally skilled to replace the guys who retire.
But that's not always possible -- in war, it's rarely possible. Instead, you have to have replacements who are green. To be more effective, they have to survive and learn -- they have to earn their stripes. All pretty basic stuff, if you've read accounts of war, whether factual or fictional, and whether it's WWII or Vietnam or probably any other war.
So the question, from a role-playing perspective, is how prevalent are veteran replacements. In my view, they should be vanishingly rare, because PC's are rare and special. That's why in my campaign, you have three choices for a replacement/midstream add-on character:
1) Start at 1st level and earn your stripes, just like every other PC.
2) Take over an existing NPC. Most likely, it's going to a somewhat weaker character, since PC's are pretty powerful, most of the time.
3) Start a 1st level character with ECL's from a "monster race". You need to earn your stripes, but your power level is closer. Of course, there are big role playing challenges to this choice.
But I think you're approaching the question from a complete different point of view. I think you're not asking what makes sense in the game world and how the world should work. I think what you're asking is what will make the player of the character who died least unhappy about that, and will have the best "game balance". You're the one who said, afterall, that having attack modifier than the other PC's would upset you -- not at all a role playing concern, but purely a gamist approach.
For the sake of generalizations, there are three types of replacements that can be made.
1) Cyclical replacements happen when you have a constant improvement of people, and by design you need a mix of new and experienced people. This works best in a macro situation, like our population in its entirety, the military as in your example, and a campaign world as a whole. It is not made for task driven small groups like adventuring parties.
2) Equivalent replacements happen when you have a need for continuity of effort, such as a project team at work, or an adventuring group.
3) Upgrade replacement happen when your task gets more difficult. This can be in the form of adding new members, or replacing ineffective members with upgrades. This really only happens in RPGs a new person joins, or a more powerful NPC is needed.
In the work world, the newby first level engineer is not compensated as well as the paragon executive. And it would absolutely suck to give both the same task.
And it would be just as bad or worse to put a much lower level character in with higher levels (assuming a combat-based game). The lower level character would be ineffective, and in a fight the rest of the party would see it as either dead wood or fodder.
It would be so simple to keep everyone at approximately the same level. Why not do it? Unless a player wants to play a lower level.
In HR terms, what we're talking about is a build v. buy strategy for talent acquisition.
You need a build strategy -- hire junior people and train them internally -- when you're in a new field or a highly specialized/rare field where the talent doesn't exist or won't change employers. To give some clear examples of situations where you need to go with "build": Hiring PC software engineers in the 1970s; companies had to hire electrical engineers or hobbyists and grow them, because that's what talent existed, since the field was brand new -- there was no existing talent pool. Amateur kids like Bill Gates were as good as it got -- some few of them turned out to, like my vision of PC's, have great potential, but there's was nobody sitting there ready to go as a seasoned PC software engineer. Or "hiring" Olympic swimmers for your country's Olympic team -- many dozens exist globally, but few can be "induced" to change countries/teams.
What I'm saying is it makes more sense to me that adventuring parties would primarily need a build strategy. In my view of the D&D world, there aren't a lot of other parties developing the talent you'd need, and there are virtually no characters who want to change parties (in a super dangerous profession, it's not good for your life expectency to work with people you don't know and trust very well indeed -- that's why it's hard to infiltrate terrorist groups and mafias -- they won't just pick up any yahoo who applies, because they want only loyal people on their six).
By contrast, I think the buy strategy (let's pick up another 9th level wizard) makes one of the following assumptions about the game world:
1) We abstract it and don't care where they came from. It's just a game.
or
2) There are multiple parties running around the area, such that it's easy to lure people to change parties and people of the right level and class are available on demand. It's about as hard as finding a 9th level wizard for your party as it is finding a good sushi chef for your restaurant in New York City -- maybe not a dime a dozen, but a talent you can easily advertise for and likely find in a few weeks. (Doesn't fit my campaign, but possible).
or
3) There are multiple unemployed adventurers without parties. Again, doesn't fit my campaign, but it could make sense -- either sole survivors of near TPK's (implying even more parties are around) or just a generalized high level environment (the next guy you meet is a 9th level wizard and "he looks trustworthy").
Oh, and BTW, if you do go with a "buy" strategy, you often need to give inducements to get top employees to switch. It's often a bit more expensive than "you look trustworthy, join us" to get a Major League closer off the free agent list, or a CEO, to join your party instead of one of the many others that exist and train up your talent for you. So, if you have a "replacement comes with levels" view of your game, the party should have to pay the new PC's a signing bonus or offer a better share of the treasure to get them in the party. Otherwise, why wouldn't they stick with folks who raised 'em?
