That's an interesting perspective. I remember opening up the Expert set in 85 or so, in which the town of Threshold and environs was described, along with a large number of quests on which to go. I also remember playing sandbox games, running into hints we couldn't handle, and therefore heading off to go get extra XP elsewhere so that we could come back and defeat whatever dungeon or mobster had stymied us.CRPG designers came up with it while working with a different medium, or mage took it from some fringe group of tabletop players. "Do some side quests to level up" is not a phrase I've ever seen published in older paper RPGs (including modules and including Dragon magazine for D&D). The whole concept of "side quest" vs "main quest" where a side quest is done for extra levels to make the main quest easier (and where the main quest and side quest just sit there waiting for you to find back to them) is distinctly not from TTRPGs.
None of this is new, and none of it was invented by CRPGs.