It was renewed...

I forget the exact wording, but on Twitter they specifically said Discovery was doing so well attracting viewers, it was renewed.

CBS is starting a streaming service, they won't cancel their first big show when they have nothing to replace it with yet

I think it was always in the plan for the series to be renewed unless something tragic happened. All Access can't be making much from it yet because they will still be a lot of people on varying versions of free trials and discounted months. I am not surprised a lot of people signed up for it. The test is if they stay and ST:D is the primary reason they are staying. You need more than a season to determine that I believe. So even though originally I thought it could be cancelled the reality is it was never in any serious danger on All Access. Now if it were on CBS itself, that may be a different story.

Also consider this, The Orville premier finished with better numbers, it appears to be the more well liked show by the average trek fan and its current ratings have dipped quite a bit. Given the harsher reaction to ST:D I have to think it would have struggled on CBS.