MS isn't cutting consumer products, they're cutting less-profitable products.
Music streaming alone isn't profitable, at least not in its current form. If customers pay $10 a month and listen to 10 songs a day (which isn't even very much), that's only a third of a cent per song. That third of a cent has to cover hosting (on Spotify's servers), marketing, customer service, development teams, office space, legal costs, and of course, the actual licensing of the music.
I'm sure Spotify makes deals to maximize their profits and minimize expenses, and it does scale in their favour with a lot of users, but that can only go so far. Especially when you consider that most (?) of their users don't even pay, it's just ads. And they regularly offer promotions to get it even cheaper (I've seen 99 cents for 3 months? How is that even possible, when a single song costs that much anywhere else?)
I really don't think it's of benefit to MS to buy Spotify. The only reason would be because it's a well-known brand.