Should there be legitimate concern about Halo and the future of the franchise? Both in quality and popularity. A few things have to be taken into account: 

a.) Halo's Metacritic has been dropping since Reach (2010 release) & this trend doesn't look to change anytime soon. While scores are subjective there are still technical blunders in MCC and (IIRC) patches are being outsourced. Halo 5 could drop below an 80 Metacritic although I doubt it would be this Halo. 

b.) The decline of the NA userbase for the XOne will surely impact Halo 5's sales. You can't point towards the original Halo either as it was a new IP & really the only driving force for the OG Xbox. Bundles will help but again as shown in current entries Halo has been declining since Reach. 

c.) The emergence of CoD, Battlefield & the FPS genre. CoD in general has taken the casual crowd that Halo would draw in. 

d.) Bungie is now multiplatform!! 

Xbox fans: are you comfortable with 343 & their direction for future Halo games?

I'm a little worried about the sales for the game, but for quality I completely trust 343, I loved Halo 4 and i'm pretty sure i'll love Halo 5.

Halo 4 was my favorite and I really enjoyed the Halo 5 multiplayer beta so yes I am comfortable.

Halo 5 won't get under 80 metascore any time soon, even if it deserves to. The days of it getting 90+ metascores and being seen as the pinnacle of FPS games are long gone, but I expect Halo 5 to still be a very good game, somewhere in the mid 80s.

I really enjoyed the Halo 5 beta, but in general, I'm sort of over competitive FPS shooters. Battlefront will be the exception, but that will be short lived. The Halo user base has shrunk, regardless. People have been jumping ship to COD, Battlefield, and now Destiny. The hype and name alone will make it a success, but yeah, it's probably not even going to sell as much as Halo 4. It may do 5 million, but doubt much more. Will 5 million be considered a success? I have to imagine MS is putting $100 million into marketing and promotion from now until release. But Halo 5 is also a huge marketing tool for Xbox One in general.

After how 343 killed Halo Reach with their shitty playlist managing, and the horrible Halo 4.... Halo is dead to me. I just don't see how people can be excited for 5.

You must be kidding me. The multiplayer beta was just fine. It'll sell millions, and it'll have a good metacritic score as well.

halo 2/3 were the best ones

It'll do much more than 5 million lifetime. I can see 10-12 million lifetime (Halo 3 did 15 million, and the 360 userbase was much smaller).

I've been concerned about Halo 5 since Halo 4, but sales wise I think it should be okay.