That would be a stupid question. There is ZERO Chance Kelly would "supplant Keenum."
Obviously IF Keenum plays well Kelly will never see the field - yes, unless Keenum is injured.
Even if Keenum struggles, he's still obviously better than rookie Chad Kelly who has never started an NFL game.
MY POINT was about a completely different matter: what chance they will put Kelly in IF
1. The team is out of contention late in the season and
2. they have to see what they have in Kelly because Elway has to decide whether Kelly is his future franchise QB, or whether he has to go out in the first round in the 2019 draft and draft a QB.
Kind of like the debate about why they didn't put in Paxton Lynch because the team had already seen that Siemian was never going to be a long term franchise QB, and they needed to find out about Lynch.
Unless he really plays well, Keenum is a "transition QB" in Elway's words. Elway is still looking for a long-term answer at QB. Keenum could become that QB if he plays well enough and the team wins of course. I, like every Broncos fan hopes he does.
But, he could be mediocre or the team could just lose enough games they are eliminated from Playoff contention. At that point they would have to put in Chad Kelly to see what they have in him so as to evaluate whether they need to go back in the first round next year and draft a QB.
The entire Keenum experiment was an effort to AVOID doing that - drafting a 1st round QB and developing him for a couple of years. Elway would prefer that if he has to replace Keenum it be Chad Kelly to short-cut the developmental process.