No reason why fans should be upset.
If so, they don't realize that increased valuation of Nintendo shares means more resources to sustain all those games they value so much on Nintendo hardware that a significant portion of the consumer base is not playing (see Wii U sales).
Presumably, expanding into the one viable market Nintendo hasn't tapped will increase the consumer base for all its IPs. This is not to say that there will be some sort of massive conversion rate as mobile gamers buy Nintendo handhelds or consoles, but it will mean increased revenue sources from an untapped market.
This takes nothing away from Nintendo's core toy business and we will still see the same level of support for both handheld and console platforms.
The only thing that should upset Nintendo fans would be if their new mobile strategy proves so impossibly successful that Nintendo shifts its focus to mobile games over their own hardware based games. I don't see this happening any time soon.