Honestly, that's not even speculation worth having because the logic goes both ways anyway.  How many people only bought it because they watched the first, say, 2 hours on Twitch and were so wowed by the game they wanted to play it themselves?  We can't know either, but both are equally likely and therefore the risk and reward are both basically balanced out on the Let's Play front.  And I would say that few people with any intention of buying a game would spoil the whole game's story on YouTube and not buy it.  I'm watched large patches of games I never once considered buying before, won't buy now.  Cause I was never interested in it, I was interested in either hearing the review/impressions/whatever for purely curiosity reasons or because I thought it would be funny to watch, even if I would hate to play.  Games I intend to buy I don't watch nearly as much cause guess what?  I don't want to spoil them. 

This is when you gotta actually start looking at hard numbers: what were Nintendo's software sales before and after the start of their YouTube policies?

Nintendo knows and I assume if there was no clear impact, I can see their YouTube policies not changing much.

I know this: before the Content ID system and Nintendo's use of it became a thing, we were in the Wii era with great sales for Nintendo.  There have been good sales and bad sales for games during this period as in all periods for all companies.  I guarantee you the policies haven't helped, it's only limited exposure.  If I were to guess, Nintendo does this purely to get the money from the videos simply because they can.  In a company that is fairly compartmentalized, it's entirely possible that a large part of the company doesn't even know about the policies unless someone directly challenges them on it. 

Also, let me point out that what Nintendo does limits the potential community aspect that has helped some game franchises swell in size.  The Elder Scrolls is a great example of how fan generated content -  mods, videos, etc - has helped that franchise spread to a massive 20 million audience.  That's very hard for Nintendo with their conduct.

This kind of topic is kinda interesting to me. What kind of lines can be crossed when it comes to IPs, especially international IPs. We all know that most, if not all, products have an "all rights reserved" marked on their products and ability to use copyright is restricted to commentary, criticism, and parody.

Game Theory had an interesting video about fan games: https://m.youtube.com/watch?v=TiYEKDQZ5sk

And here's another interesting vid, this time from Buckley: https://m.youtube.com/watch?v=dGlnKvjLFOs

Granted these are examples for fan games, but the point still stands about copyrights and such.

It's feels like there's a lot of gray areas when it comes to using that has all of its contents reserved. I'm not saying either side are wrong and I think this bears a more deeper discussion and research. The internet has exploded throughout a short amount of time and social media has become a big thing for the ordinary citizen. Thus why we're seeing some companies do what they're doing now. That's why we're seeing music on YouTube having change of pace and pitch to avoid copyright. Or direct feeds from live shows, movies, etc.

As for marketing and such, I guess it depends. For example, Splatoon apparently didn't suffer from Nintendo's YouTube policies and there are various videos of Splatoon gameplay. Also, ive rarely seen reaction videos like reacting to Smash Bros characters, unless content creators actually point it out on Twitter or something. It's a weird situation.

Yes they don't need to cover Nintendo, as well as reviewers don't need to cover Nintendo. But all that falls more on censorship than anything else.

And you know that this arrogant attitude from Nintendo is what lost then support in N64 era right? People like to pretend Nintendo is the good company, friend of all, but they are the biggest offender in several fronts.

But have they hurt?

Well the Wii period was followed by the Wii U period.  Is there a connection?  No way to know.  It certainly didn't help.  And if you can confirm neither, than is it worth the considerable legal exposure?  I mean, if Nintendo wants to be a hardass and be super strict with copyright control, I accept that.  That's not what they are doing.  They are using Content ID to circumvent the legitimate legal channels by which they *should* be handling copyright control.  This use of the system could easily lead to a lawsuit against Nintendo if they piss off the wrong guy and they would have little defense if it's a case like the ones with the Cooptional Podcast.  So if this is gaining them nothing of note, why take that risk?

If a lawsuit occurs then so be it. Nothing wrong with giving a multi-billion dollar company a boot up its ass from time to time.

Because it can make them money.