There is a lot going on here so I'll just go post by post. In the first post, the dual wielder talent does not provide stat bonus damage to an offhand weapon, only a fighting style does that. That talent is required to dual wield anything other than 2 light weapons however. But your point stands on the bonus attack feature, so pretty much any class can do it, but without the fighting style they lose the stat bonus to damage.
\Monks have basically no access to armor or shield magic items, but they can use any simple magic weapons or magical short swords without sacrificing any of their abilities, so I'm not sure what scaling you're talking about. If it's magic, well that is the person that creates the campaign. My group doesn't use a lot of published adventures but if I remember right the only magical item that rogues can use and monks can't in Lost Mines is a +1 long sword. I also recall that adventure having a ring of protection which monks can use. But it just serves to highlight that the ball is in Larian's hands how they stock the dungeon.
With flurry, yes you won't be using flurry of blows all the time, but you still can use your bonus action to make 1 unarmed attack if you use the attack action, so you reliably have the option for 3 attacks a round from 5th level every round. And the fact that you can regain all your ki points on a short rest means you are far less restricted in how you spend your ki, particularly in a hard fight.
As far as weapon enchants, most only affect the a targeted weapon so you don't gain the benefit on a dual wield attack. But yeah, divine favor and crusader's mantle require a weapon so you only get the benefit on your attacks made with a monk weapon.
As far as gold goes, it really is going to be up to Larian if they are going to provide a shop that sells magic items, but in the few published modules my group has played, there were no places to buy magical items. If you go by AL rules, well you are limited to scrolls and potions for the most part. So again I'm not sure how this benefits rogues or monks.
A first level fighter will generally not have plate armor, it's 1,500gp for a set in the PHB. Plate armor provides 18 AC, a shield provides +2 AC so a fighter wearing non-magical plate and shield will have an AC of 20. And yes, that is pretty much max level for a monk, but monks aren't supposed to be front line fighters in the same way as fighters, paladins, and barbarians. Monks are far more analogous to rogues and rangers in that sense.
As far as magic items, monks really don't need any magic items to do well, but can benefit from a ton of them. And it really depends on the campaign you are playing, the low magic campaigns I've played in made monks extremely valuable, but they performed very well in high magic campaigns as well.
With the monk/rogue example you are right that I added the magical bonus of a +2 short sword to the bonus attacks, so a non-flurry attack chain would average 37 damage and a flurry attack chain would average 44.5. And I didn't include the option for the rogue to make a dual wield attack which would add an average of 3.5 for 37 damage and it could go a bit higher if the off-hand weapon is magic. But the point is to show that these classes have synergy and that the damage output is similar, but they offer very playstyles. And while you are correct in that the monk has more attack rolls so more chances of failure to hit, the monk also has more chances to land a sneak attack and the monk/rogue can use stunning strike to gain advantage to all attacks on a stunned enemy so in that way the monk has options that straight rogues don't in that they can engage in one on one combat and still utilize sneak attack. Again the point of the matter is monks have a lot of synergy. None of their abilities require unarmed attacks, but the bonus action attacks are always unarmed.
Honestly I don't see monks as having the problems that you are suggesting unless you are trying to shoehorn them into an always unarmed attack mode which is not monk RAW or RAI. I think they have synergies with multiple classes making them good multi-class options with the exception of the MAD nature of monks. But ultimately this is only my opinion, and YMMV.
