While I would've loved to see I-70 completed to I-95 as planned, in reality there's just no good path that can be taken without ruining the park first (or the neighborhoods immediately bordering it). As it is, the planners should've made more modifications to the route so that it would take out less parkland but leave the neighborhoods intact, which was damn near impossible even back then. However, I think it was a good call building I-170 below grade, because a viaduct would be even more damaging to the surrounding areas that were declining anyway.
Then again, I-95 was supposed to have gone completely through DC at one point, and look how that turned out. Exact same situation as I-70, but through far more built-up areas.