There are many reasons why it would displease me. There are two main reasons I can give as a bit of an abridged version.
I’ll preface this by saying that this is my own personal opinion.
The biggest reason is that the covenant races are beings in the afterlife that work with the life death cycle. It doesn’t make sense for beings who use to be any living creature on Azeroth that have duties such as ferrying recently deceased souls to their final destination or would work to redeem beings who were evil in their life to suddenly join a random faction war and go off adventuring. Now, if they were not in the afterlife and didn’t have such duties, I’d be more open to it, but as it stands, they should stay in the Shadowlands, as it diminishes the impact of death if multiple groups of people die and comes back to the living world as a Kyrian or something.
The other big reason, as I said before, is that Blizzard has mentioned considering all races with two legs for allied race status. If true, the idea that they decided against a lot of playable races, many of which have been requested for years, to make up beings in the afterlife and have them become playable instead would leave a sour taste in my mouth. This isn’t to say making new races is a bad thing, since we got a lot of awesome races like Vulpera and Sethrak. Even Saurok were a brand new race to the franchise when MoP launched. It’s specifically the nature of the covenant races, their coming out of nowhere, and them having more involvement in future storylines that would really just muck things up and, in my opinion, would leave a general “bleh” feeling as opposed to being a hype announcement.
There are also races part of the Horde and Alliance that are part of those factions and still not playable. Not to mention several races that have been requested to become playable that would both be very well received, add in a lot of flavor to character customization, and would serve long-term for potential storylines and characters. To choose beings who are dead and should have little reason to care about a random kerfuffle between two factions to become playable over them would just be very disappointing to me.
I could go on with other reasons and in greater detail, but I’d likely be writing a thesis paper on the whole subject. Honestly, I’d rather see living races people have been wanting for years to become playable actually get on that character creation screen. Ogres, Saurok, Sethrak, San’layn, Arakkoa, Kelfin, Saberon, Jinyu/Ankoan, Hozen, Forest Trolls, the list goes on. All of them potentially adding much more to the game long-term and making a lot of people happy than beings who effectively would treat the world of the dead as a revolving door.
Again, all I typed is my own personal opinion. I would be foolish to state they is no chance of covenant races becoming playable, but I feel like the chances would be close to being zero percent. If they did become playable, it wouldn’t cause me to quit, but it would really start steering me towards that direction if new race choices would be handled haphazardly in such a manner.
And besides, that character creation screen could really use some reptiles on there 
But reality is very different.