"You go straight along that path."
This is the explanation I found on another question like this:
すすむ: to advance, to proceed. If there is a route, one probably advances through it. In such contexts, this verb is preferred.
行く: to go. If one says to go to work, go to school etc., there is only a target or purpose.
Why not そこの道... ? It's really hard to understand how to use these almost-identical words そこ, そっち, そちら etc, when you never know if the reason why your answer is rejected is because it is wrong, or if it is just Duolingo team being rubbish at keeping multiple valid answers accurate. People spend time reporting errors FOR FREE for Duo, yet nothing happens. How friggin hard can it be to actually go through the reports and add valid answers to the database? Damn slacker website... >:-(
I second your concern over そこ, そちら, and そっち. From what I understand, そこ is the most exacting - its there (pointing to a specific spot), where そっち and そちら are more vague - its that'a'way (pointing in general direction), with そちら being the most formal (some say its more formal than そこ, others say its on par or slightly below). All should be accepted, generally speaking.
As far as "slacker website", I don't think you realize that DuoLingo doesn't create courses. Its merely a host/tool for volunteers on both sides (teachers & students) to use. The teachers or course maintainers are also completely volunteer based, who work not only on fixing errors, but are constantly adding new material and other updates. DuoLingo has 0% involvement in this process outside of merely being the platform on which everone volunteers. Also, you get what you pay for. Duo is free, and it has some issues, but it shouldn't be your only source of learning. Even though you're paying for PLUS, you're paying for additional access to the platform, not for the language, so again, not Duo's problem. You woudn't yell at Twitter if someone told a lie in their tweet would you? Same concept here.