A test in noPassthrough provides very limited test coverage, whereas tests in jstests/core are run with all the configurations / fault tolerance modes available in our passthroughs. They will also be picked up by any new suite that is added in the future, so in that way they help with future coverage as well.

If this is too much work for CS alone, and some of this work is outside CS's scope but still covers sharding, consider filing a ticket for the routing team