Yes I would mind, because of many reasons.One of them being that Nintendo is the most unique company in the whole industry, and most of the time they are the ones that push the industry forward.And if they stop doing hardware, well, the industry will lose one of its pioneers in innovating(hardware wise).
But of course there are other reasons, like pride.Nintendo always was(for most part anyway) the developer for its own platform, and i want it to continue like that because i like how things are.And also Nintendo games are so good because they are on a hardware they made, so they know how to take the most of it.And well, lets face it, if you ask someone who is fan of Sony/Microsoft if they would mind them going third party, their answer would be yes, because they want their favorite companies to succeed on all fronts, hardware included.
Nintendo keeps everything a whole lot more interesting. If there wasn't an underdog to root for, one that did something different from the competition, we'd all just sit here watching the PS4 outsell the XB1 for the 9004th week in a row.
Nintendo is that company that is unique, different, plays by own rules and bringing huge diversity to gaming with they own hardware and software. They are company that have new crazy, great or bad ideas. You can say whatever you want about Nintendo, but fact is that are very interesting and no one for sure cant say what next will do.
Also going 3rd party would definitely effect on quality of Nintendo games. People dont realise that one of reasons is why Nintendo games are so great is because when Nintendo making new platform they always have their games on mind, basically they making platform that will make most use of their hardware and that will suit them most. When you have your own platform, you making rules, you play by your rules, you can do whatever you want, basically you have full control. All that is big part of reasons why Nintendo games are great and have great quality, so going 3rd party would definitely affect quality of their games.
If Nintendo would go third party basically that wouldn't be same Nintendo any more, nothing would be same ever again, that would be beginning of end Nintendo.
But I think that questions like this are very unnecessary because even with Wii U Nintendo is making money and this gen is already in profit. So there is no real danger that Nintendo will go 3rd party, and it seems with NX Nintendo is moving from traditional concept of consoles, I think much bigger chances are that MS will abandon console gaming for instance.