What you were witnessing was not debate. Speaking to a largely empty chamber is the norm, and serves a different purpose.

Debate and discussion of legislation happens in the discourse Senate members have with each other, with their staff, between their staffs, with committee staffs, and with private industry and other lobbyists. These discussions are happening all the time, just not usually on the Senate floor. So while it does seem weird that Senators orate to an empty chamber, that doesn't mean that the other Senators aren't hearing the arguments.

Whether they are hearing the right arguments, however, is a different issue.