My take on this while not a horrible decision, it is a weak one for MS. My rationale;
1. 70% of the world doesn't care about their browser and will use what’s is put in front of them. There was that classic video where people didn't even know the difference between the internet and a browser a few years back. For this 70% unless someone swaps then to another browser, which thankfully people love doing, they'll use IEX for the rest of their life. These people have been trained to click the blue 'e'. Why take that away from the masses and open the door for them to look for alternatives?
2. For the remaining people that do care about a browser, we're going to know if it’s a great browser or not. We understand IE6 is not IE11. In the same was FF was awesome a few years back, we know it’s fallen back, and would recognise it improving if it (hopefully) does.
3. Microsoft announcing they will create a new & awesome browser, and actually doing so, are not connected. It wouldn't surprise me if MS invest all this marketing money on spinning a product that is just as frustrating as IE, just for a new set of reasons. And now we have brand confusion + an annoying browser with a bad name. People prefer familiar bad, than unfamiliar bad.
4. Microsoft is a bloody big and wealthy company. They can afford developing 2 browsers for a while. Why not launch and release this Spartan browser 'for power users'. They can use it as an experimental ground to push boundaries whilst not risking the stability and working of their core product. As this team proves their metal, they can push components of Spartan into IE, or not if the browser fails to deliver. Spartan can be used to buy new street cred for MS. Mum uses IE but young me uses Spartan. And if spartan succeeds by default it will bring credibility to IE, risking much less if it fails to gain favour. As a marketing guy rather than building the conversation; Spartan vs Chrome I’d much prefer to have IE vs Spartan vs Chrome.
And if you bothered to read this far, you probably like browsers. So you might enjoy a video a buddy and I made while on holiday for some really serious (/s) browser speed testing: https://www.youtube.com/watch?v=zaT7thTxyq8