No. A Rogue only needs Dex. There is no expection of needing anthing else as they are not designed to stay in melee, use heavy armor, or roll ability checks outside of Dex.
Then he will be lacking in combat because he will be limited to inferior light armor. A Rogue who wants to excel in combat anywhere close to other martials is going to need both strength and dexterity and will also need medium armor proficiency and the last comes with stealth penalties unless he further uses a feat.
This is to bring him close to what the fighter starts with. A fighter comes with heavy armor and better hit dice and does not need to invest in dexterity at all.
Fighters are combat machines and are intended to have their prmary and secondary be a combination of STR/DEX/CON unless they are EKs or AA then your prmary and secondary is INT.
If you want to confine your player to that stereotype do it, but don't act like this is the only way to play the class.
I said that background and class are both expressions of the same concept and if you think noble is good enough, then the acolyte is good enough as well.
No they are not the same concept. A fighter is not a noble and a cleric is not an accolyte. Background and class are entirely different things.
If holywarrior can be 10% or 50% of your character concept, then aristocratic adventurer should be as well.
A holy warrior is only part of your character concept if you make it your character concept. If you want to be that then be that, if you want to be an aristocratice adventurer be that. It is totally and completely irrelevant to the class.
Only 1 class carries that kind of baggage with this class itself, that is the Warlock which as a Patron and necessarily affects the kind of character you are.
Easy Checks?
That's you problem right there.
Medium DC is 15.
Major game changing checks are not supposed to be easy.
You falsely claimed such a character would fail over half his checks, that is not true and a character with no bonus and no proficiency would fail half of the easy checks.
Your DM is running Easy Mode. That's why you think noble fighters are fine.
I have played with numerous DMs and I have DMd official modules. A medium check is a medium check and it or easy are the most normal check made.
Make an easy acrobatics check to walk across the homemade makeshift bridge or fall off. This type of easy and medium check are by far the most common.
Moreover when it comes to those hard checks, such a character will succeed a substantial amount of the time, 5 times as much as someone with no bonus. With proficiency and +1 at 5th level he will suceed 25% of the time at a difficult check as compared with 5% vs a character wit no bonus and no proficiency. By the way that Wizard who dumped everythign else to get a 16 Charisma and proficiency will still FAIL over half of the difficult checks.
If difficult checks are your benchmark NO character can pass over half of them at 5th level unless they have an 18 in the ability AND have expertise in the skill. Even a Rogue with an 16 Charisma and expertise is still not above 50% for difficult checks at 5th level. So if this is your benchmark your original statement is meaningless - they are all failing "most" of their difficult checks.
DC 10 as the most common DC? WHAT? I must have miss something in the book.
If you are mostly rolling Easy check, your game is easy.
Example DC 5 checks:
Navigate in grassland, meadow or farmland, repair something
Examples of DC10 checks:
Net, Ballbearings, stabilizing a creature, forced march for 8 hours, chart a course, check to avoid exhaustion after 10 hours in extreme heat with no water, check to avoid exhaustion after 1 hour in extreme cold, navigate in most types of terrain, tracking on a soft surface, foraging with plentiful food and water
Example of DC12 check:
Average contest
DC13 checks:
Hunting Trap, hide against the average CR5 foe, find an average hidden monster in the monster manual (I am guessing this is about 13)
DC15 "medium" checks:
caltrops, lock, not becoming exhausted from going without water, forced march for 13 hours, spot a disguise being used by someone else, catch a player cheating, forge someone's handwriting, determine your location on a nautical chart, repair an item in half the normal time, navigate in forrest, jungle, swamp or open sea with overcast skies and no land in sight, tracking in dirt or grass, foraging with limited food and water
Now there are all specific examples of checks that are medium and below, most of them are explicitly called out in the PHB or DMG. Why don't you provide examples of all these "difficult" checks your party is doing regularly. Ones from the PHB or DMG would be extremely useful (I know of only 4 in the PHB and DMG combined).