Took some time to think on this, and I would say that vampires in general are fairly popular. As such, I do think San’layn would be decently well received, and not just because I particularly want them.
They’re definitely a niche that would add in some great flavor to the game. Not a lot of MMO’s have done vampires for the most part, other than Everquest 2 having a vampire race and ESO having a vampiric status off the top of my head, so it would be a fairly unique addition. And with San’layn in particular, you have some pretty great storyline opportunities to work with them as well. I mean, the door pretty much opened for them in BfA. It’s just a manner of having them stride through it in style.
And yes, there will be people who complain about it. Then again, people will complain about any race becoming playable for whatever reason. There isn’t one race that could become playable that will please everybody. Regardless, I see San’layn being decently popular. I mean, people were cheering for them in that one Blizzard Twitch stream and speculation about them in BfA seemed fairly positive from what I saw.
As I said before, WoW has a pretty good opportunity to offer several different types of races to fill in many niches that would be welcome additions for those a fan of a certain race. Vampires for San’layn, Saurok and Sethrak for reptilian races, Ogres for big brutes, Kelfin and Ankoan/Jinyu for aquatic races, Saberon for a feline race, Arakkoa for an avian race, and so on. Lots of potential variety that would work well for marketing buzz. As well as attracting fans of those niches.
And vampires in particular are still going very strong. Give them great backstory and storyline, and I think many will warm up to them. Not to mention that releasing playable San’layn at a point where vampires are getting hype in general would likely be a popular announcement. Again, with some people grumbling, but what else is new?
If it would be that much of a worry, then put them in a group of playable races like with BfA’s announcement. They’d likely be even more popular in a group with other requested races. It would be pretty hype to see people’s dreams come true at once with several races becoming playable.
We’ll have to see what happens in the future, whenever we’re done with the Shadowlands. Hopefully, it’ll involve some popularly requested new members of the Horde and Alliance joining. Not a guarantee, but I’ll always keep hope of some kind. A lot will really depend on what direction the next couple of expansions take us.
I feel like I rambled quite a bit, but hopefully I conveyed my opinion decently. 
