This all sounds logical on the surface, but I don't think it pans out like that. Once Republicans stop winning elections, Democrats stop using that as an excuse to move right to court centrists and disappointed Republicans.

Yes I know, this is your fictional baseless nonsense you keep repeating as if nobody has responded to it, including what I just wrote and that you haven't addressed. We already have examples where Democrats have won consistently and they didn't do this. You have examples right in front of you at the state and municipal level and Dems fight tooth and nail to prevent universal healthcare at the state level, they constantly bump funding for cops, they send cops after protesters, they cleave to Israel and use every bad faith excuse to go after pro-Palestine organizers, going after their schooling and careers, they implement austerity for social programs and dehumanize the unhoused. At no point did using the logic of always voting Democrat achieve anything, it is actually disenfranchising to think that way and tell others to do so.

Once Republicans stop winning, two things happen. One, the immediate contest moves to the primary, where everyone is most directly vulnerable to their constituents since turnout is especially low for these.

Primaries are controlled by the parties in power, as is ballot access. The contest "moves" there in the sense that multi-party democracy is dead on its face in those places, as the Democrats can exclusively focus on punching left in the interest of their donors and connections. They focus hard on excluding even left liberals like Greens. They then move to trying to control those primaries to keep out challengers as best they can. Redistrict to remove left politicians, for example. And don't forget, the party is not a public thing, it is a private venture that can change its own rules basically however it wants in most cases, arbitrarily moving qualification thresholds to remove challengers. Do you think cutting off ballot access for left challengers is a symptom of perma-elected Democrats moving left? Do you have literally any experience trying to make something happen electorally? You have clearly never realized who your full set of political enemies are if you want, say, to end the genocide in Palestine.

If Republicans are crushed in the election instead of narrowly losing, Democrats thinking about reelection have to make extra sure they're palatable to the base because that's who's going to sink them and replace them in the primary.

In reality they actually try to engineer the primary so that they do face the less popular Republican rather than a slightly-left-of-them Democrat, Green, socialist, etc. Depending on the structure of the primary of course, that is their ideal. You have surely already seen this in how Dems fight hard to engineer contests to be not between themselves and a milquetoast socdem (e.g. Clinton vs Sanders) but instead be between them and some braying racist (Clinton vs Trump).

I have already pointed out that palatable to "the base" is antithetical to them and that it isn't even really a base, it is their left flank that they string along. Maybe you belong to that group, the only reason I qualify is that it's unclear you have any left wing politics at all. "The base" is who they manipulate into supporting them (you're trying to do that yourself), not who they actually cater to in policy. This is a basic lesson you should realize about the party: they don't deliver for their "base". They fight as hard as they can to avoid doing so. That dynamic is the most important one for understanding the Democratic Party and what it truly is as a political project. It's why they seem "incompetent": they don't actually want to do the slightly left things that would be popular, delivering material benefit to their people, because most of those policies would come at the cost of bourgeois interests. You can go and join the party and watch that dead end in progress if you'd like. See how much time they spend on donor lists and dinners and trimming planks and undermining primaries and conventions.

Two, since a first-past-the-post voting system always inevitably results in a two party system, if the Republicans prove themselves truly nonviable, the party dies, and another takes their place.

The Democratic Party wants a viable Republican Party. They literally say this openly.

You are trying to describe a realignment in the form of an entire party displacing another. This hasn't happened since the American Civil War. Do you have any unstanding of why that hasn't happened in 150 years? What about the bourgeois electoral system prevents this from happening even though FPTP allegedly makes it locally quasi-stable? Surely there should be examples of rupture at the state level, right? Why not? Who benefits from this not happening? What function does kayfabe opposition play?

The new party is obviously going to have to move left since right has proven itself nonviable, and so the contest moves towards left wing ideas.

There are left parties that have tried this. What is their experience? Do you even know.