And yet, not everyone adheres to that. Nintendo very famously doesn't. EA can charge what they want for the base game and the DLC and people can pay for it or not.
At a certain pint, I feel like if people really wanted it, they'd have it. It's 2015. Perhaps you're not as interested in Mass Effect 2s DLC as you think? (Not you specifically, diaspora.)
Both Nintendo and EA
do drop the prices of games and DLC over time. Mass Effect 2 and 3's DLC is the outlier, not the rule. The market
does expect that games depreciate over time.
The problem with ME2 and 3 is the perception of EA charging $10 for
3 full games while also charging $15 for DLC packs.
edit: I suspect the reason for EA's refusal to put the DLC on sale or restructure its delivery is due to the content being inexplicably tied to Bioware Points. The only Bioware game to not use Bioware Points (Inquisition) goes on sale on a fairly regular basis, has a GOTY edition that also goes on sale while the standalone expansions get 33% cuts regularly.
Consequently, if we expect Andromeda to follow the same system as the last Bioware game, it'd make some measure of sense to expect it to follow the same trends.