Obviously nobody knows what's going to happen in 8, for all we know a second Starkiller could kill everybody five minutes in and the rest of the trilogy is about their clones. Yes Finn largely does have a character arc in this film by all accounts however unlike Rey's, his seems to be mostly done. He became disillusioned and at the end of the film has a strong reason to fight. Now that doesn't preclude him from partaking in whatever action or scenes, but even the limited amount of attention he got in this film seems likely to be shifted further into Rey and her training with Luke. Both of whom are the stars of this trilogy.
It sucks shit that minorities thought they were going to get a co-star in a leading scifi action trilogy. Boyega was getting asked all the time about how it feels to be a Jedi in interviews so it's not like Lucasfilm didn't know what was happening. Instead he's getting relegated to the usual black sidekick according to some reports. Now that might change in 8 but there is no reason to assume that will be the case. In fact, him being put on ice in-between films seems to strengthen the notion of his irrelevance.
I assume almost everybody didn't expect Finn to beat Ren or whatever, however things like him being in a coma also limits growth outside of the actual movies. Finn also not being Force Sensitive limits his possible importance to the trilogy as well when you have all these super Force Users running around. That's not to say he won't have a part, but it's certainly doesn't appear to be anywhere near the level built up in the marketing. It's not the biggest of deals, but it does sting.