The link title is misleading. MS will not have 5k people working on AI. They merged their MS Research and their AI engineering team. It's not like they are going to drop all the other research they are doing to focus solely on AI. The new org is called the AI and Research Group. This is just a reorganization to democratize some of the resources available within MS.
This seems good strategy. I am no expert, but AFAIK Microsoft research division is maybe (the) most advanced and most achieved research division in whole industry. Just look at people with H-Index more than 30 who works in Microsoft research.