That^

I know this is a speculation thread but the chance that this ever happen (Microsoft backing out) is probably as low as Final Fantasy 7 remake coming to Xbox :)

Microsoft is a company, a really big company. When such companies decide to purchase another, the first thing they do it analyze if it's a good investment or not, and MSoft decided that purchasing Acti-Blizzard for the prize they offerered was a good deal from a business perspective. Yes, it could have been cheaper now, but it will also be cheaper for someone else if they don't finish the job now.

Taking that into account, there is no chance MSoft will back down from this unless the relevant authorities decide to step in and put a lot of conditions to greenlight the deal.