Winning a game 1-0 is the same as winning it 5-4. Yes, the starting pitching has been good recently, but it could be better, just like the offense could always be worse.
It doesn't work exactly like this, and I know I'm kind of playing with things a little bit, but what if Javier Assad didn't start in Toronto last week and Edward Cabrera did, for example? The Cubs lost 5-1, sure, but four of the runs were given up by Assad. What if we got Edward Cabrera's start against Atlanta (8-8) instead? He went 8 IP against Atlanta, struck out 11. Owen Caissie was in Iowa on August 12th, so he provided zero benefit to the Chicago Cubs.
There is also going to, likely, come a day between today and the end of the season that Kyle Tucker turns mostly back into Kyle Tucker. Maybe it's not "140 wRC+" monster Tucker (if it's really an injury thing) but he'll probably figure it mostly out again. At that point the Cubs will likely rarely play Caissie where as Edward Cabrera will pitch every five days.
Lots of things and lots of moving parts. Maybe Ian Happ blows out his quad tomorrow and Caissie starts the rest of the year. Maybe Cabrera blows out his elbow. There's universes and outcomes that throw all sorts of wrenches into the works.
Mostly I'm just saying I don't think that just because Owen Caissie was good yesterday means that not trading him either makes the Cubs better or that the Cubs were right to not trade him. There's a lot of moving parts to consider.