Well, part of that is because as the US isn't a parliamentary system, progressive parties aren't really able to get anywhere unless they get absorbed into a larger, more centrist one. I think the UK is somewhat of an outlier with Labour being as lefty as it is, largely thanks to Corbyn, which is also why they're having trouble getting anywhere. But Labor in Australia and the SPD in Germany are both fairly centrist, left-leaning-but-not-insane parties. That nutty goodness is able to express itself in smaller parties, who may have a shot at getting into parliament (later, at least, as right now it's dawn of the smaller conservative parties).
Dems electing Perez just means that there's going to be more and more internal strife because he's of the same blood as Obama and Clinton--the party's old guard. He's not likely to give into many of the progressive demands the other side puts forward, and the other side is unlikely to try to maturely leverage their political power against him... so there's going to be a fit, more infighting, more all of this shit.
That said, comparing Ellison and Perez, I think Perez has the chance of giving the party some gains in 2018. Ellison was without a doubt a slave to the identity politics nonsense, which would faithfully ensure the democrats rake in loss after loss--but if Perez is able to garner popular dissatisfaction with Trump and offer a ~reasonable~ alternative (as opposed to 'fuck white people and let's make everything free without any idea of how we're going to pay for it except 'fuck people rich enough to just move their shit overseas at a moment's notice") to the new Republican political pole, he could make some minor wins. Or at the very least, staunch the bleeding of the party.