I'd say in Kylo's case it was more that he was being teed up to be the villain by Johnson - in his interactions with Rey, his selfishness and willingness to exploit others comes through in spades, and so it makes perfect sense when he takes the throne. Which is something Trevorrow ran with, and seems to be yet another thing Johnson and Abrams disagree on.
I think a lot of people have heard about Trevorrow's script second-hand - Trevorrow's script has Ben redeemed before he dies as well - its just way worse than Abrams/Terrio's script. It very last minue and quite unsatisfying.
I'm always personally surprised at the belief that Johnson was setting up Kylo to be the villain of the trilogy with what he was doing in TLJ - to me it was always obvious from TFA that Ben would come back before the end (his being killed off and robbed of the opportunity to meaningfully atone was depressingly predictable and yet still
incredibly frustrating given the other choices made). Nothing in TLJ changed that for me at all - it just reinforced it. A lot of people placed outsize importance on Kylo 'rejecting' an 'offer' of redemption in TLJ, but that's totally irrelevant. It's the second act of a three act story*. Luke telling Leia "no one's ever really gone" is the actual message Rian is sending - and that's reinforced where we see Ben inside the Resistance base - he's still the conflicted quite depressed villain, even at the end.
TROS following through on that is one of the few things it actually did pick up, correctly, from TLJ, IMO.
*It'd be kind of like arguing Zuko is irredeemable because of his betrayal of Katara at Ba Sing Se.
But yeah, the teasing and use of Finn as a decoy protagonist/Jedi is something which riles me a lot. As far as character arcs go, the way Finn is treated in TFA suggests that Abrams had no idea beyond mere affectation. He wanted the texture of a rebel Stormtrooper, but couldn't think of anything to use him for.
Yeah I've said it before, but TFA loses interest in anything to do with Finn's background pretty much immediately. None of his background really makes sense for how his character behaves, and it never informs his motivations. Sadly, TLJ continued this - but with some justification, given how Finn is not at all interested in fighting for the Resistance at any point in TFA, even by the end, and the story arc TLJ chose for him was about getting him to that place.
Trevorrow, though, bizarrely, picked up on this point in a real way. TROS completely shit the bed.
The deceptive marketing campaign was just egregious. This is the second time Abrams has been involved with this (Khan with Into Darkness, anyone) and it should never have been allowed to happen.
Choosing to make Kylo co-protagonist rather than villain also leads to a bit of what I see as TLJ's character bloat, not helped by the multiple separate storylines. It's a bit like they are trying to give him a prequel story, while still having him be Vader, but not really having the time or balance to pull it off.
Kylo Ren has slightly less screentime in TLJ than he did in TFA, though. It's really got nothing to do with making Kylo co-protagonist in the sense that TLJ did it. It's like Boyega says in the interview - his character lacks nuance compared to Daisy and (especially) Adam.
The problem is Kylo Ren being one of the big characters - and always likely to overshadow Finn - is baked into the premise. He's not only the villain, he's also the Skywalker, the son and nephew of all the legacy characters. The story was always going to be predisposed to his gravitational pull more than anyone elses. It's not an either/or thing anyway - they could've done better with Finn's character without taking away anything from the others.
In so far as screen time is concerned, it certainly wasn't Kylo Ren who took Finn's time. It was ... Poe. Who was barely in TFA, and who was actually never meant to survive TFA in the first place. I mean I love Oscar Isaac but ... the movies didn't need a 'trio'.
TROS' attempt to force one in the last installment was terrible.