How do you estimate the Xbox One would have fared if they hadn't fumbled so disastrously out of the gate, and instead:

- It was $400 from the get go

- It was on par with PS4 in performance

- No bundled Kinect

- No "always online/no used games/TV TV TV" nonsense

What are your thoughts?

I doubt it would have been $300 especially with power on par with PS4, which was $400 at launch. I could see $400 at launch without Kinect, no always online controversy, and a better design that looks less like a VCR. With that I think the generation could have been a bit better for them and likely outsold PS4 in the US. 

Xbox's biggest issue was first-party games as at that point they really only had Halo, Gears, and Forza. If Xbox did the acquisitions a decade sooner they could have had Bethesda games exclusive like Skyrim to keep the momentum going from one gen to another. And of course Fallout 4. Xbox could have kept paying for Call of Duty marketing rights, but they decided against it with the launch of the Xbox One. 

Without the always online debacle and $400 launch price I could see the Xbox One having sold another 5 to 10 million or so, with lifetime sales of ~65-70 million. With acquisitions a decade earlier I could see the Xbox One selling more on par with the PS4, much like the PS3 vs 360.

Now if Xbox One released a year earlier in 2012 for $400 with Halo 4 as a cross-gen launch title that would have given them a decent lead by the time the PS4 released. 

Xbox has made a lot of mistakes over the decades. You could say it started with the OG Xbox as Rockstar approached Xbox first to develop GTA3 for it, but it was turned down.

Whoops that was a typo, thanks for catching it! I meant to write $400.

As an XB enthusiast, I've always wondered this myself. If it matched everything PS4 had, I think it would have reduced the NA market share loss, to then translating over to this era. I think it had an exciting E3 (minus the TV part and always online stuff). I still had a lot of fun that era! More than this one at least!

It would've helped for sure, but not night and day at all. It definitely outsells the 61.91 million of NES. But that's a low bar to clear considering Xbox One sold 57.96 million.
In your hypothetical scenario, Xbox One still gets the S and X and Game Pass becomes a thing. And backwards compatibility probably still gets added in 2015 in this scenario like real life.
I don't think launching with backwards compatibility would've been say more significant than a $400 price tag, no Kinect, and none of the controversy with the reversed policies.
Very little chance Xbox One could hit 70 million units.

Sony only announced $399 to counter MS's $499 but without all the outcry at Xbox in general I'm not sure Sony keeps the PS4 price at 400 bucks. Maybe they do. Then considering Xbox technical limitations. Japan not wanting to support it and Phil in charge. In the long run Xbox One is in the same place it ended up in anyway. Mostly mediocre exclusives and some decent games but nothing that is a system seller. That's pretty much why Phil opened the checkbook to buy studios Their lineup was subpar and they needed games that sell. The bad launch only really hurt MS the first few months or so. Look at PS3. Horrible launch but they recovered and ended strong. A bad launch does not seal your fate automatically.

I think the PS4 would have sold less than it did and the Xbox One would have sold better, Honestly if you look at what Phil Spencer did for Xbox during 2014-2017 and if he was able to do what he did in 2012 just 1 year before the Xbox One released it would have blown the PS4 out the gate. Backwards Compatibility day one, dumping Kinect, more focus on studios / buying studios before launch, Game Pass at launch, a better designed Xbox One console outside and inside (more like the One S), not use the Windows 8 OS, etc. If this was all there in just the first year of the Xbox One I wouldn't see why you'd go with a PS4 unless you're just a PS guy.
Remember if the PS4 didn't sell as well as it did that would have been the end for Playstation, so if Microsoft didn't fumble (and same with Nintendo) Playstation as a console might not be here right now.

I think it could've reached the 70-75m range and retained a big lead in the US putting it in a far better position. A huge aspect long term though is first party support since if the issues there remained then the Xbox Series would've still continued the decline regardless.

It would have done a lot better and perhaps MS would never have started releasing all of their titles on PC.

Sony was already moving more strongly into the Chinese and Middle Eastern markets. Even with these changes, MS would've still suffered from their poor localisation efforts.

The situation would be better for them no, no doubt.