Again, I'm not saying they weren't going to fire him at the end of the current season. But firing him mid season is always a worse financial choice, unless you have someone to immediately take over for him. That's because, as with any abrupt firing, you can be liable to owe your employees severance and have to pay out part of their remaining contract.
The only way what you are saying will make any sense, is if they get out of the late night business entirely, and don't replace Kimmel with anyone. It's possible. Late night TV shows don't yield the ratings they used to across the board.
But assuming ABC wants to keep their late night show and replace Kimmel, this wasn't planned. Because they'd either fire him mid-season and instantly replace him with someone else (didn't happen), or they would fire him after the season and replace him with someone else.
Logic doesn't lie.