I keep hearing how "we need a viable conservative party." Why? What for?

[Azathoth]

57. ...

they might leave the GOP to form another political party

Sure, but this effectively just reestablishes the traditional political landscape by replacing the GOP with a new mainstream conservative party and once again quarantining the Bircher freaks, this time within the tiny remains of the rump GOP. Unfortunately, it's highly unlikely to happen as a practical matter, for the same reason it's unlikely that moderates or progressives and socialists would split from the Dems: they know they'd be splitting their coalition and consigning themselves to irrelevance for the foreseeable future. Most people aren't stupid. They realize it's better to be locked in an alliance where they may have some tiny influence, rather than locked out in the cold where they will have none at all. The only way a meaningful new party is going to emerge is if it can quickly pull enough support to reach some level of parity with its opponents.

that hasn't been true of federal income taxes

Well, two observations. 1) That's a reasonable claim, but it's by no means an accepted fact. 2) If we assume you're correct and federal taxes aren't too high, then you have to consider that fact in light of conservative counterpressure for the last half century. Can we say with reasonable certainty that they wouldn't be too high right now without all those decades of conservatives fighting to lower them?

the tax burden has shifted downward

It certainly has.

There's nothing magical about the private sector

No, but the point is that when one side reflexively looks to government to solve every issue, and other reflexively looks to the private sector, and both sides are rational, you will (hopefully) get a competitive process where the most effective solutions eventually emerge based on demonstrated superiority.

Look at the difference between how Democrats responded to Senator Franken's alleged misdeeds

Yes, and also look at the absolute rage here on DU and elsewhere in the Dem base directed at those Democrats. It's no exaggeration to say that Gillibrand's chances of being on the 2020 ticket pretty much ended after she turned on Franken. The impulse to enforce party loyalty and overlook the misdeeds of those who you agree with ideologically is not exclusive to the GOP.

Incidentally, I'm not arguing that Franken did anything he needed to resign over. But let's take a more troublesome case: Cuomo. Anyone who wasn't a complete party apparatchik knows, and has known for some time, that he screwed up big with his nursing home order. But since there's really no effective, responsible opposition party in NY anymore, there was no serious oversight to hold him accountable in real-time when it might have mattered, and he was allowed to bluster his way through until the press forced him to change course, and then he was allowed to conceal the effects of the policy and stonewall legislative inquiries.

I honestly didn't see Democrats abandoning their oversight responsibility during the first two years of the Obama administration, did you?

Well, as I said in my previous post, we have to be careful here, because Democrats objectively are orders of magnitude less depraved and contemptuous of our constitutional form of government. So no, nothing the Dems did during Obama's first two years are even within the same solar system as what the GOP did during Trump's first two years. But let's be brutally honest: I don't recall too many hard-hitting oversight hearings and waves of subpoenas of Obama officials being issued circa 2009-2010.

I don't see any real argument for the continuation of a national conservative party, aside from, as you say, giving conservatives a chance to rant and feel listened to

Considering that self-identified conservatives still outnumber self-identified liberals/progressives by around 10 points, they could easily make the same argument about having a liberal party.