The senate has the right to set the rules they follow. A majority of the senators can chose to allow their will to be thwarted by a minority. The minority knows not to abuse the privilege, because the majority could get tired of it and change the rules.
What is constitutionally suspect is requiring a supermajority to vote to change the rules. This "rule" was based on a concept of the senate as a body that never ends -- so there are standing rules. It is argued that because each 2 years only a 3rd of the members are voted on, the majority serves through each election so there isn't really a "new" senate.
But no court would accept the argument that 33 just-elected senators can be bound by a rule adopted when they were not there.
BTW, this does mean that the Republicans would have been on firmer footing had they simply changed the rule for judicial nominations at the start of the session. But in fact there was no rule passed at the start of the session, they are merely abiding by the old rule, so they haven't (according to republicans) given up the right to change the rule. The democrats in the past have argued that the majority NEVER gives up the right to change a rule, even when they vote to require a supermajority to change the rule.
But what the republicans are GOING to do is challenge the filibuster on CONSTITUTIONAL grounds. The filibuster isn't likely constitutional OR unconstitutional, but the courts will leave the determination of constitutionality to the Senate, which can make that determination on a majority vote, which is how they will sink the filibuster.
That all just sounds like blah blah blah to me. The SCOTUS currently composed doesn't care what is or isn't constitutional. Why on earth would the Senate ask them? They should just push their nominees through. This whole argument is pointless. The GOP look like fools. Arguing constitutionality with liberals is like arguing human rights with Iranian mullahs. What the hell do you expect to accomplish? And it's not unconstitutional to filibuster anyway. Instead of squawking for year after year after year, they could just take action. I am beginning to think they are all frauds.