I believe this has been said already, maybe in other ways. But, from an organizational stand-point, you do not give your plan B a try unless you are ready to make the decision that your plan A isn't going to work. 'Not working' is not the same thing as 'not going to work'.
The Colts have to be 100% ready to walk away from their current QB if they think they'll sit a healthy Brissett to let Chad Kelly have a start. It possible the move could spark intense competition that makes them both better. Its happened before. But its not the only thing that has happened before. You could also destroy a budding confidence who may never recover as well as trespass on a fragile eco-system that is rooting for 'their guy' (Brissett's allies, aka 'the locker room').
In the football world, the business world, and personal relationships; you just don't start looking to see what you have in a new thing, if you aren't ready to destroy the old thing. You have to be done with it. It is my belief the Colts aren't there yet. Not for Chad Kelley, at least. The draft, that's different - that's just business. But sitting your starting quarterback just to see if the other guy is good enough, could/would damage the faith and confidence Brissett is going to need to become the guy we all need. If there is any chance of that remaining, and for sure there are glimpses of that, then no way does a competent headcoach (nor a front office) allow themselves to undermine Jacoby Brissett.