I think you're making magic item more common than intended by the text, while this choice is shown as the DM's choice by the book, not the expected standard campaign. 13th level characters in the campaign I currently run all have a magic weapon, but nothing more than +1. They don't suck and can handle the challenges quite well. Nothing breaks. I felt I was running it as intended... and to be honest I don't think my group deserves to be laughed at for playing this way. Especially since it's supported by the rules.
The DMG does says, as you mention, that "a character doesn't typically find rare items before around level 5", but it doesn't mean that they typically find a +1 armor at level 5. Saying that one typically doesn't become president of his country before age 40 doesn't mean people typically becomes president at 40. After level 5, it's possible to find them, but it could still be a noticeworthy event.
To support this position, I'll quote another part of the same section : "most magic items are so rare that they aren't available for purchase [...] common items, such as potions of healing, can be procured [...] Doing so is rarely as simple as walking into a shop and selecting an item from the shelf. The seller might ask for a service rather than coin". While I could understand your reading of "typically" finding items taken in isolation, I get from the remainder of the text than magic shops are inexistant (barring of course DM intervention) even for lowly healing potions and other common items. I agree that most players would be nonplussed if the famed relic they found in the tomb of a holy man was a common potion of healing, but I think there is support for a default game were magic items are quite extraordinary. In these worlds, even the so-so list of artificers infusions can shine.
Of course, you could have more magic-rich world but, to keep quoting the DMG "Magic items might be for sale in bazars or auction house in fantastical locations such as the city of Brass, the planar metropolis of Sigil or even in more ordinary cities". Basically, if you need to get to the elemental plane of fire to trade rare item like a +2 sword or a +1 armor, having the infusions is worth it. Having them available in large cities is a deviation from the intended play, and I wonder why it is supposed to be bad faith to mention that they represent a good setting for artificers to thrive.
Let's look at the actual expected distribution, since we get hard and fast rules for those and not just general DMing advice.
According to the treasure finding rule, adventuring groups can find +1 weapons on table F, +2 weapons on table G, +1 armor on table G, +3 weapons and best +1 armor on table H and a few +2 armor and +3 armor on table I. Over a full campaign, the make :
7 rolls on the 0-4 CR table, with 12% chance of making 1d4 roll on the F table and 3% chance on the G table.
At this point, they have 32% chance of not even having rolled on the relevant table. Rolling does give you 15% chance of weapon +1 and 11% of a +2 weapon on the G table and another 10% of +1 equivalent weapons. So that's on average 0.31 +1 weapon from the first table and 0.052 from the G table and 0.057 +2 weapon. Put it another way, you could find a +2 weapon before level 5, which would make artificers pale, but the odd of it are a little under 2%. The odd of having no magic weapon at all are 68%.
18 rolls on the 5-10 table, with 11% chance of making 1d4 rolls on the F table, 4% on the G table and 2% on the H table
After the middle of their adventuring life provided they don't end their campaign here, the odd of having no magics weapon are much lower, but still 17.8%. On average, they would get a little over one magic weapon at this point (for the whole group), when the artificer can assuredly provide a +2 weapon. It is its apex, though, as he won't be able to get +3 weapons through infusions. However, the remaining odds on the :
12 rolls on the 11-16 table
8 rolls on the CR17+ table
Will just guarantee that everyone gets a +1 weapon, not a +2 one. So I definitely think that the playstyle where everyone gets his own +1 weapon around level 5, at the tier barrier, isn't the "default" playstayle against which the game is balanced.
You mention Xanatar's advice to be "generous" with magic item. It's true it is written in XGtE, though the quote is a little longer "Magic items can go from nice to necessary in the rare group that has no spellcasters, no monk, and non NPCs capable of casting Magic Weapon. Having no magic makes it extremely difficult for a party to overcome monster with resistances or immunity to nonmagical damage. In such a game, you'll want to be generous with magic weapons or else avoid using such monsters." I don't read this sentence as an incentive to be generous in all case, only in the extremely narrow case described just above. I have, in my experience, never met a group without magic users. I won't say they don't exist and maybe are even common, but I feel they are a deviation from the "intended average" against which the magic item rules are balanced.
The rules Xanathar provide for given items, but over the 1-10 levels of play, a group is only supposed to acquire 7 uncommon major items and 1 rare major item. If you respect the roughly 15% distribution of magic weapons among major items, that's a 1 or 2 +1 weapon among the whole group and maybea +1 armour... that are the staples of the artificer's infusions. In the extreme case of a party without spellcaster, where the DM is incited to be generous with magic weapons (but not advised to change the overall number of magic item), the 20 major items the party will find over its adventuring carreer wil be just enough to equip them all with +2 weapons and armors.
