Early, buggy release = fewer giveaways
Remember how MS promised that upgrades to W10 would be free to W7 and W8 users "for the first year after release"?
Releasing the new OS in a, frankly, barely-beta-worthy condition, may be seen by some factions in Redmond as a way to keep the takeup down, and thus encourage more people to... pay for the product later.
Yeah, it's stupid and it'll damage their already-tarnished brand still further. But because of the factionalism and infighting within MS, that's how it unfolds sometimes. This sort of passive-aggressive "compromise" is exactly how the brand became so tarnished in the first place.
Right now, I'm running Windows 8.1 - and contrary to all my expectations, I love it. Easily the best version since XP, beats the heck out of Windows 7. It'll take either unanimously stellar reviews, or the promise of a substantially enlarged support window (8.1 expires in January 2023 - extend that to 2027, and we'll talk) to persuade me to upgrade to 10.