I think you've hit a couple good points here that deserve to be expanded on. Maps may be the best source of resources per time spent, as long as you aren't picky about what resources you get. Low-level players need everything, so the random distribution of rewards from maps doesn't seem negative to them, and they probably don't have quick enough teams for PvP to make maps look bad in terms of time/payout ratio.
High level players are likely to value certain resources less than others, such as souls. They still need keys, gems, and traitstones, but guild tasks are better for keys and gems, and they're more likely looking for specific traitstones, so Explore mode is more efficient. Guild tasks raise the value of gold astronomically, and PvP is the best source for gold. Once guild tasks are reworked, other options might become a little more comparable.
So even if maps were the best source of resources, high-level players would still likely value maps less than other options, because they don't value all resources equally. But on average, they already are a highly competitive source of glory, which all players need pretty much equally, because it's used for things both new and old players have the same amount of: new troops. And traitstones are still new enough and rare enough that few people, if any, have an abundance of the most valuable: minor reds and arcanes of all colors.