Okay, that part makes sense. But the flat check is still a little weird; it's not easier to sneak up on a baby than a regular person? It's DC 15 for everyone?
Hide action = DC 15 and you're invisible feels gamey, that's why people are mentioning rerolling. If someone gets a 15, what's stopping them from saying "eh, I don't think I'm hidden well enough, can I try again?" Rolling first does create odd things.
They are standardizing the checks, to show that the default DC is 15. It is 15 for influence and 15 for stealth. IF you want to make it easier, go ahead, no one will stop you.
And the thing stopping them is the rules stating that the DM calls for the check. You call for the initial check that they roll first. It represents them starting to sneak in. You don't allow people to start a negotiation, stop and say "eh, I don't think I'm negotiating well enough, can I try again?" so why would you for Stealth? They performed their actions and started moving, they don't get a do over.
But it is harder to hide for a long time and do multiple things while hiding. So someone happens to roll a 20 and as long as they stay behind people or in obscurement they're basically invisible forever? A 1st level Rogue could have a +7 Stealth bonus easily. Since they roll a Stealth check to use the Hide action, if they get a 20 they know they beat anything with a passive perception of less than 27 (or 22 with advantage) and are golden for ... how long? A minute? An hour?
If they are still stealthing, why not? Investigating a room can take up to an hour, and is a single roll. You don't re-roll investigation for every piece of furniture, or if you spend a week researching you don't re-roll for every single book. A single check can cover a longer period of time.
And you can still have them re-roll after something significant alters. If they stab someone, they need to re-roll. If they end up getting to the room they were looking to break into, and investigate to tear the room apart, then as they leave, they need to re-roll. But if they roll really high, and they are just scoping the place out and don't do anything else... it was one narrative action, so why force them to make seven rolls?
This could be handled if Stealth is rolled when you try to do something stealthily. It depends on what kind of rounds you're in. Combat? That's per round/action. In a dungeon? Per 1 minute of exploration actions. Traveling? Per 1 hour of traveling actions.
You're guaranteed to miss some attacks sometimes. The rogue is gonna knock over a vase or step on a creaky board eventually too.
Right, but if you miss in combat, it doesn't immediately lead to you losing the combat. If you fail a single stealth check, then you have failed the entire stealth mission and need to either fight or flee. IF you were in a combat where missing the enemy a single time meant death, then you would be essentially guaranteed to lose, and that is what happens when you turn a single narrative action "I want to sneak into the camp and find the prisoners" into seven checks, each one more likely to fail than the last (statistically) and ever more dangerous for the character who likely is moving solo because that is how scouting works.
Seriously, rolling a natural 1 on a d20 is a 5% chance. Over the course of three rolls, the odds of getting
at least one nat 1 is 14.26%. The more times you force a character to roll a check, the more likely they are going to fail. So we should not encourage rolling again and again and again and again. "Oh, you moved from low security area to high security area?" Okay, that makes sense to re-roll. "Okay, you hid behind the box, then stepped out and slipped behind the guard, then moved into the room by the entrance, then exited into the hallway, then waited for the next patrol to pass... that's five stealth checks please" is not reasonable. It is setting the character up to fail.