First of all, the kid is 16.
And? Still ignorant.
Secondly, telling a customer that you won't make them the game they want because they can go play a 13 year old game instead (one who's profit margins are basically zero at this point) is a somewhat suboptimal business strategy.
If we're interested in honesty, then the child is simply wrong and ignorant- and (politely) referring the child to the already existing merchandise is a good idea simply to correct the misconception.
If we're interested in business strategy, returning to a settled issue that has long since been moved past, simply because of kid's request, is an even more suboptimal business strategy. Referring the child to an existing game, however, would be an optimal strategy because the child is new revenue for an already existing product, no additional cost to the developers. Whereas creating a massive new game returning to earth is not untapped revenue... because this child was already buying the not-at-earth Mass Effect games, and isn't credibly going to stop buying them or lead a consumer revolt after Bioware spends a decade of a franchise not-at-Earth.
Thirdly, I can't believe people think a series that invests so heavily in the the not-so-distant-future vibe (N7=Special forces, Systems Alliance = United Nations, etc.) will never make another game in or around Earth again.
That's a pity- but fortunately, your belief is not required.
The reasons why not to return to Earth are the same reasons to leave the Milky Way- which, by your own scenario, stood longer than the Mass Effect trilogy even existed in the Milky Way. Games after ME3 struggle to cope with the endings without 'invalidating' anyone's route, games set during or even before the Shepard trilogy are tied in by canonical constraings and questions/issues of wanting to tie in Shepard's trilogy. (Think of the people who've complained about the Ark theory- now try and convince that brand of logic about the super-important spinoff plot.)
You can certainly make games- spin offs and spin offs- but the trade-offs have apparently indicated that Bioware doesn't want to stick around Earth.
Whereas the elements you raise aren't tied to, or require, the Earth: N7 will be N7 in Andromedea, fictional international talk-shops will exist, and none of them depended on any significant role for Earth in the original trilogy's narrative.
Mass Effect was never a particularly strong 'near future' political set-up after ME1- ME2 ditched the politics angle for daddy issues, and ME3 had the Reaper War. Human politics (like the UN) were largely irrelevant, and the inter-species analogues can still exist.