I can fork Bitcoin tomorrow. So can anybody else. The only matter if whether the fork can gather enough traction to have value.
In particular, whether it can gain enough mining power to survive. Currently, SegWit2x (the fork discussed here) has 92% of miners signalling that they'll support it[1]. The question in this case is whether the original Bitcoin could survive after the fork if this support is maintained. [1] https://coin.dance/blocks
However, algorithmically speaking, miners consensus _is_ Bitcoin. The only authority Core developers have is rooted in tradition. So far, it was enough to get miners to agree with their point of view... but it will be challenged eventually.