There's a basic fallacy with that: it was not the Senate's constitutional duty to "fill the spot" with Garland. The Democrats did not have the votes for a straight up-and-down vote, never mind avoiding a filibuster. If it was the Senate's "constitutional duty" then they would have been violating the law by not doing so. Moreover, as you likely know, the Democrats had promised to do the same thing, the so-called "Biden Rule."