Keep in mind Mearls has said some items are intentionally tucked away.
Meaning you should not expect to find all of them. Viewed like this, the amount doesn't feel overly large.
Regarding the "Ability 19" items, well, as you yourself are admitting, ability scores of 19 is well within reach of low-level characters, especially if given their level four stat bump: start with 16, choose a +2 race, and bam, you're already hitting the ceiling (20) at fourth level.
So my concern isn't that these items will unbalance anything. My main concern is how much easier these items make certain builds.
That is, if you knew you were looking to get, say Dex 19 at fourth level, this would open up so many builds even with mediocre stats that it isn't funny.
The biggest "problem" is therefore the randomness of which abilities are covered by these items. In one adventure, such as this one, one party member can ignore Constitution, since if he or she survives this far, he or she is set for life with Con 19.
But in another adventure, a dozen levels might pass by with Int 19 items or Wis 19 items etc etc and not a single Con 19 item in sight. And for a certain party, that Int 19 item can be, well perhaps not worthless... but definitely a "second tier item": the Wizard is already sporting Int 18 well on his wat to Int 20, and nobody else has focussed on any skills. In such a group, an Int 19 item would... well, not be thrown away, but the ranger or cleric ending up with it won't exactly see new avenues of character growth with it, and may well ditch it when they come across their fourth attunement item.
Contrast this with a Dex 19 item for a dragon sorcerer who had to focus on Con and Cha, leaving Dex at 10.
Or the adventure item (Con 19) for a barbarian who took Str and Dex and didn't roll more than a 12 in Con. Not only does this item open up the cool Unarmored Defense feature, it might at 6th level give him a cool +18 hp... Point is: this is infinitely more desirable than a Cleric getting a +3 on his Int saves, and getting above-average Int-based skills... for this particular group.
Since there does not seem to be any magic item economy (making items, trading for items) in 5E our hope is for the DMG to include advice for the DM to ask for "wish lists" from his or her players, I guess.
