The first part is true. We diverged from the path when old laws turned into sacred things that could not be debated, questioned, or changed. "Because we've always done it this way" is a dangerous mantra.
The second is feeding off the idea that because they didn't fix everything they were truly evil, yet many of the same problems existed before and after them to this very day. So we're not that much better and shouldn't judge with such absolutism. Hell, if they hadn't taken action in their time against oppression and tyranny it's hard to say what kind of world it would be now. I wonder what it was like to be at the edge of authoritarianism and have to make a choice between ignoring it, coping with it, embracing it, or fighting it. Good thing we don't have those problems now, huh?
Point is, if you're going to judge the past, be sure to judge it from its own relative viewpoint and not from centuries later, and definitely not in a few one liners that would make grade school history look like graduate school level. Slavery is a big one that's used against them a lot. What if some of them were full abolitionists that disrupted the Revolution efforts in order to push against colonies that used slaves much more than others? One, they wouldn't have succeeded, and two, the colonies wouldn't have united against a common foe.
And who knows, maybe that alternate future does work out after a century or two in some other way, but it's a far cry from just a simple "why didn't you stop slavery while you were at it?" There's reasons why some battles were chosen over others at that time. Some progress is better than none, or worse, regression.