Uhh, they are patching **** for MP though? Every single patch notes has some **** for MP, most of the patch notes right now are just crash fixes and memory leak fixes, except for the 1.1.0 yesterday we really haven't gotten new features either, and 1.1.0 was more of a balance change than new features.
You're acting like SP has already gotten tons of new content and MP is still left in its early form, which is not at all the case. I get that you want the aspect of the game you like being the best it can be, but just read the patch notes and you'll see that SP isn't really getting much content either.
Someone else also already mentioned that SP is the core of the game, most people play SP. Doesn't mean they shouldn't focus on MP, but indeed you should appeal to your majority.
Keep in mind its still in early access, they're producing patches literally every single day. Have some patience, they'll get around to MP when the CORE of the game is working fine with as few crashes as possible, then they will get around to making new content for both SP and MP. If you decide to refund it (assuming you're still within the 2 hour limit) that's obviously completely up to you, but you bought an early access game, you always take risks when doing so.
Lastly, I want to quickly mention how ridiculous it is to threaten with a refund unless the developers do a thing you want them to do. They don't care about your 45 bucks if you're gonna act that childish. You're being incredibly direspectful towards them with the threat too, they are putting a TON of daily work into this game to make it as best as they can as quickly as possible, and you don't even have the patience to wait.