Because the core issue of class balance centers around this question. "The party needs X role filled. What class can best fulfill that role, as well as bring along other utility?"
I disagree.
The question should be "The party needs X roll filled.
How can the classes we have best fulfill that roll, as well as bring along other utility?"
You never
need a Rogue to fill a role, just like you never need a Wizard, Fighter, or Cleric, or any class to fill a particular role. Some do it better than others depending on the role, certainly, but that is where it ends.
And if the role is "Scout ahead, find and disarm traps, and open locks" the ONLY thing that would make consider a full rogue over a caster like wizard or bard is something on the level of Reliable Talent.
Again, I disagree. Most games don't even make it to tier 3 when Reliable Talent comes online (2014), but due to having more skills (4) and four expertises, Rogues can do a wide variety of taskes effectively even before then.
Bards are a whole other issue as full casters, which they should never be IMO, but even they will only have two expertises prior to 10th level, not the four a Rogue has by 6th (a more commonly played level). IMO, Bards should never have Expertise. The class should be more about the Jack-of-All-Trades, Master-of-None concept.
I would much prefer my Wizard party member or my Bard to focus on other things than have them deal the the things the Rogue can easily handle.
I'm not arguing this other classes can't do it, but given everything else, they really shouldn't be doing it.
Rogues aren't useless or unplayable or anything like that. They can carry their weight. But other than the rogue imagery, they're mostly just OK even at the core rogue tasks.
Again, I have to disagree. Rogues excel at rogue tasks, even before Reliable Talent. That might not be your experience, but as someone who plays rogues a lot and has a rogue in nearly every single party I've run or played in, that is certainly not my experience!
Whereas I remember the days of AD&D where my Thief often failed his stealth rolls because AD&D did not actually want you to succeed. The percentages started too low and there were so many penalties that you could not even do your class function until you were at a level that magic had supplanted you anyway. I would rather a rogue be a little too good at their job than be a useless waste of rations for the majority of their career.
AD&D is a differnt beast. Let's not move goalposts, ok? Thanks.
In 5E, a 1st level Rogue with Expertise in Stealth is +7 or +8. Most passive Perceptions are 10-12 at that level. Meaning you can sneak by them 80% of the time or so.