Well, it's not this straightforward. I'm not sure where you got your numbers and percentages from, but working on the assumption that they are correct:

It also matters where the votes come from. If the remaining 40% is all clustered in California and New York, and they all are convinced to go out and vote, and also vote blue, that doesn't have an effect on the outcome.

Why? Electoral College.

See this chart, showing that split ticket voting (in particular, folks who voted all Republican EXCEPT for the President, where they voted Biden) was about 4% out of everyone who voted in 2020: https://www.pewresearch.org/politics/wp-content/uploads/sites/4/2020/10/PP_2020.10.21_split-icket-voting_0-01.png

Also, look at the small margins that Biden won on the swing states according to https://www.npr.org/2020/12/02/940689086/narrow-wins-in-these-key-states-powered-biden-to-the-presidency. So what this could mean, assuming that the 4% holds generally, is that the 4% difference cause by split ticket voting is greater than the margin that Biden won in those swing states. So without winning those folks - those moderate Republicans - over, the Dems lose the Electoral College and thus the Presidency.

The thing is progressive policy gets the votes Harris is chasing here too

When Obama flipped red ststaes in 08 it wasn't because progressives turned out, because progressives always turn out.

He flipped red states by flipping moderate voters with progressive policy.

While moderates have favored the Democratic candidate in each of the past five elections, Barack Obama gained the support of more voters in the ideological “middle” than did either John Kerry or Al Gore before him. He won at least half the votes of independents (52% vs. 49% for Kerry), suburban voters (50% vs. 47% for Kerry), Catholics (54% vs. 47% for Kerry), and other key swing groups in the electorate.

https://www.pewresearch.org/politics/2008/11/05/inside-obamas-sweeping-victory/

And that's even with the PUMA movement Clinton's campaign started to get Dems to vote for Kerry.

https://en.m.wikipedia.org/wiki/People_United_Means_Action

Even if we lose some votes, progressive policy is a net gain.

Because progressives are probably smart enough to know why they need to vote for Harris instead of letting fascism keep them on their couch

How about you go after the 40% that sit between the two groups by making it worth their while to get off the couch and go vote?

They can't or won't because their bread is buttered by the same people buttering the bread of Republicans: rich donors and SuperPACs.

All the "progressives" in Congress endorsed her. Guess that's not enough, eh?

The evidence is in the fact that every republican who has come out to support Harris is retired from politic life either by choice or by having been voted out by the rabid faction that makes up the vast bulk of the republican base.

Your evidence shows that Republicans whose careers no longer depend on Trump are the ones who can be convinced to turn on him. The same is true of many (formerly) Republican voters.

This is stupid. How about you make a play for disenchanted progressives who actually could be convinced to vote for you?

Democrats would rather lose democracy forever than treat progressives like constituents with agency.

Progressives who dont vote for Harris are even dumber than Republicans. Not sure if talking to them would help since they would need to stop focussing on breathing just to listen to you.

This is just plain wrong. Vox says it better than I can,

It reflects wishful thinking and a rigid set of political priors — namely, that Democrats’ political problems always stem from insufficient motivation of base voters — more than a cold, hard look at what the electoral and demographic data say.

Source: https://www.vox.com/the-big-idea/2018/1/29/16945106/democrats-white-working-class-demographics-alabama-clinton-obama-base

Also, keep this in mind,

Campaigns tailor their messages for swing voters, who are not demographically representative

Source: https://www.vox.com/the-big-idea/2016/11/22/13713148/electoral-college-democracy-race-white-voters

The campaign of Harris knows what it is doing. This, sadly, is indeed what it takes to win.

"Disenchanted Republicans" aren't the fascists. They're the ones who don't want to vote for the fascists, but have been swayed by fifty years of Republican propaganda to be uncomfortable voting for Democrats.

If you want to de-polarize this country, reaching these people is vital. They need to start realizing they have to vote for ideas not parties.