As far as the justification: It’s even more obvious with this process than it is with filibusters in general that the issue isn’t so much about “minority rights” as it is about empowering individual senators. That makes sense; individual senators don’t really care as much about “minority rights” as they do about their own personal efficacy, whether they care about that for re-election or simply because they want to be able to affect public policy.
Which gets back to the question of what constitutes “abuse” — not in some abstract sense, but in a way that senators will care about. All senators have both party and individual concerns. For those in the majority party, those concerns may well clash when it comes to Senate procedure; as Democrats, Harry Reid and Pat Leahy and the rest would prefer a smooth, streamlined dictatorship of the majority party, but as individual senators they want to preserve the rights of all senators (the same, of course, is true of Republicans when they are in the majority). “Abuse” from that point of view happens when the balance between these two impulses gets out of whack, with the minority party using those rights of individual senators as an excuse for what is effectively minority party control. The practical line where majority party senators are more concerned about majority party control compared with individual senator rights depends on lots of things, including the extent to which the individual policy (and even reelection) goals of those senators are satisfied by enactment of their party agenda.
What we can say in the “nuclear” episode was Republicans were clearly going over the line, but collectively either they didn’t notice or they didn’t care. If it’s the latter, expect it to happen again with blue slips and other forms of obstruction. If the former, then perhaps they’ll be more careful next time.