Lets say you do a plus 4 that drops 850 gear , 840-845 is required for a one chest run, and its only 2 pieces of gear, whereas a PL run gives 4-5 pieces of gear per run. If you want to have 2 chests you better be at least 850, which is the same level as what drops. So basically you go in there hoping for warforged or titan forged gear but Whats the rate of those, 10-20% for warforged and less than 5% for titan forged is what i guess.
You fail to account for the fact that mythic + has no weekly lockout, and I would hardly say you need 850 for 2 chests, 2 chests isn't terribly difficult, it requires fast pace and the basic knowledge of knowing in what sequence to push the 3-4 buttons in your rotation.
M+ doesn't have a lock out. If you wanted to target said instance you could find people with keys and run it over and over again.
It has to have low amounts of loot drop because there isn't a lockout. Raiding is overall more efficient, but M+ has a lot of potential, just more time spent. If you're at the gear level where 840 gear is still good, and you're still running low M+, it's likely you would want to base mythic anyway as it's a pretty efficient gearing method.
Outside of that it's meant that you go up in the levels to get better loot, otherwise you roll the dice on TF/WF/Socketed loot (which is what everything that gives loot in this expansion does anyway).
At first I thought 2 pieces of loot was too low, but it's honestly fine. Could see them buffing it to three, but even as it stands it's a great source of AP and is a pretty efficient use of time if you can get 2-3 chests each run. It's repeatable content as long as you can find keys.