The thing is, it's Conker in Project Spark or no Conker at all. If they were really using him just to sell their (free) game, then they could've chosen any character in Microsoft's repertoire, which admititaly isn't that vast, but under that school of thought, they still would've gotten more exposure with, say, Master Chief. But they didn't choose Master Chief, they wanted Conker? Why? Because they're fans of Conker.
Outside of the occasional Minecraft skin, Conker was as dead as you could get. A new Conker game was obviously not on Rare's radar, and it obviously wasn't on the broader Microsoft radar. But out of no where, Team Dakota wants to pay respects to Conker. Even with that very generous choice of theirs, they easily could have just thrown Conker assets in and called it a day. But no. Team Dakota decided to practically make a full Conker game when no one else desired to do so and no one at MS was asking for it. I don't know about you, but I think that's incredibly cool.
Tl;dr: Be grateful.