Then all we're going to do is get more Trump because clearly "Nothing will fundamentally change" has somehow not convinced people to keep voting Democrats.
Why do you think Agema wanting the left to actually develop proper policies and think about the implementation and how to tackle difficulties means he wants "nothing will ever change".
It seems quite the opposite to me. He seems annoyed that left parties, even when they occasionally get to power, have nothing to actually implement that has more substance than wishful thinking.
And, as he is British, he has probably other examples in mind than the US democrats. For example Labor who finally managed to currently win an election and now, well, managed to lose a lot of voter goodwill.
Or Frances left block whose suggestion to the current, dire financial difficulties of the country is to spend even more because it "will generate growth and thus even more revenue". (The same stupid argument that usually comes from the right : there it is "lower taxes will generate growth and thus more revenue")
Or Greeces Tsipras gouvernment which basically went with "We will end austery and can do that because lenders will just forgive our loans". Varoufakis is an utter moron.
There are a lot of leftwing gouvernment that come into power when a country has severe problems. And then not only fail to solve those problems but also to fulfill their other election promises - which nearly always are pretty expensive.
Now the populist right is even worse.
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Now for the US specifically : There has never been a third party coming from the outside, growing and pushing an established party away. New parties only came to be when an old party imploded completely (like the Federalists) or split (like the Democratic-Republicans).
If you want to change things, you have to change the Democratic party. Either from within or via the many primaries on every level.
It is pretty much useless to vote third party on a presidential race when third party can't even get a single state governor. It is functionally identical to spoiling a ballot. (A protest vote is different from not voting, no one can assume apathy or lazyness)
And yes, the US democrats should move to the left. Even among centre-left parties across the globe they are esceptionally far to the right currently.