I thought it's kinda low too. Especially considering Black Ops 1 360 alone had nearly 3 million players at launch.
This was on GAF, saw it earlier.
https://www.youtube.com/watch?v=IWPMCFL151E EDIT

EDIT: I know the PS360 had a much bigger install base, but the stats still seem a tad low.

All 1.5 million of you :p

As much as I'm enjoying the multiplayer, 4 maps for supremacy and walker assault is pathetic.

With the 10 free hours with EA access, I'm surprised the Xbox One version wasn't on top.

1,499,999 million.  I'm kinda disapointed with the multiplayer, I don't think ill be playing Halo 5 much more at all.  I'm going back to Destiny.

Those numbers are really bad for pc. Ps4 currently has 5 times more players playing.

X1 is devent even if EA access helps a little.

Those low numbers are good. That means the young generation of gamers are expanding their taste in gaming.

It's no longer shooters all the time.

These numbers when looked at alone merely imply one of two things 

A) Battlefront 3 underperformed sales wise.

B) Battlefront 3 is having a tough time retaining its user base.

However, when you look at the sales of other first and third person military type shooters there does appear to be an overall decline in sales. So where are those sales going? This is the gen of the RPG.

Fallout 4 just destroyed its predecessors launch sales. It will undoubtedly surpass them lifetime.

The Witcher 3 just destroyed its predecessors launch sales. 

Dragon Age Inquisition was met with critical acclaim this gen.

A 'niche' RPG known as Bloodeborne was met with critical and commercial success.

So looking at the bigger picture, you're probably right

That's lower than I expected, and Xbone number is closer to PS4 number than I expected.

Those are pretty terrible stats across the board.