I think that xbox one will loose even more mind share this year when compared to the PS4 due to this move. There are many types of Console gamers, people who care about exclusives and people who don't, but it's clearly better for a console to have them than to not have them.
If anything I think the move will give more attention to these Xbox titles in the console space as more people (whether console gamers or PC) will get to play them and therefore talk about them.
And again, we are talking about console exclusives going to PC too. Not console exclusives going to another console. That (the latter) would cause actual and/or far more damage as (again) the majority of console gamers only care about what's going on in the console gaming space.
You seem to base your argument on exclusives don't sell anymore because console gamers don't care anymore, they just buy consoles to play COD/FIFA/GTA, all supported in Halo underperformance.
I didn't say this and neither am I basing it solely on Halo 5. Exclusives do sell but they matter less this gen than last gen (and the gen before that for that matter). Due to this, on top of most console gamers only caring about console gaming, many will continue on seeing a console exclusive as a true exclusive.
Most console gamers simply want to play the most popular games that are solid. Not "what's a true exclusive" -or- "first party vs. third party".
If we see Uncharted 4 sell 8M+ copies would you change your stance?
Within what amount of time?
How about Mario Kart 8 that has an attach rate of nearly 50%? And Splatoon, a new IP that sold 4M+ on an install base of 13M~?
You realize that you brought up something that goes against your point right? Wii U is the oldest current gen console (by 12 months), has some good selling exclusives but has sold less than the Xbox One and PS4 therefore showing that the majority of console gamers simply want to play the most popular games -- the majority of which are multiplatform thanks to them being available for more users to play.
Exclusive content is a pilar off the console business, and I believe that by cuting it of you will be basically puting a foot out of that business. But allas, we'll know the result in 2~3 years.
I just don't think games going to PC causes much damage if any at all. Multiplats (most of which are available on PC) are pushing current console sales the most and "console exclusives" will still be viewed as true exclusives by the majority of people who play on consoles since they have no interest in PC gaming.