1. This team is not going to win anything this year, making the playoffs could be a stretch given how they have played lately. The bullpen is a mess and not worth trading good young players just to patch it up for a short lived run if they make the playoffs
2. They have gotten some decent pitching prospects back in these trades. Killian came with Canario in the Bryant trade, Ben Brown was the return from David Robertson last year, and Wesneski was acquired from the Scott Effross trade. Those may not be "ace" caliber pitchers but they are still young and developing and good returns in those trades so far and could be a part of the rotation going forward.
3. You aren't going to win every trade you make but the sell off in 2021 we are seeing several of those players produce in the minors and even some knocking on the door or already getting a taste of the big leagues. So far I think they did pretty good getting talent back for guys that were rental players and not guys the Cubs were willing to pay top dollar for. Can you imagine the mess they'd be in today if they had signed Rizzo Baez and Bryant to expensive long term deals?
4. Too early to call Mervis a bust and they signed him as a non-drafted free agent so the fact he's killed minor league pitching and is getting a shot in the big leagues now says something. Even if he does bust they didn't trade anything away to acquire him
5. The Cubs aren't going to resign Stroman, Bellinger, and Hendricks so you want them to just keep them all season, trade away guys in the farm system for some short term help, and try to make a wild card spot then let those 3 guys go for nothing in the offseason? This would not be a full sell off like the 2021 this would be more like 2014 when they traded Samardzija and Hammel to the A's and got Addison Russell. The Cubs were halfway competitive that year but not good enough to make a playoff push and had some guys they weren't going to be keeping around when they built that 2016 WS team. The 2014 team finished 73-89 and some fans weren't happy with that trade initially but Russell was a big part of that 2016 success before all the issues happened with him.
There are going to be options to land an ace for the rotation this offseason or even a solid top half of the rotation guy at worst. This is the UFA class this offseason:
2024 MLB Free Agents Tracker | Spotrac and there are others that have some kind of option that could be available too if you adjust the filters. I just don't see a reason why the Cubs can't be selective sellers this year and still keep the core part of the roster intact to add to this offseason and as top prospects make their way to Chicago. Things could be much worse, look at the mess Oakland is in right now for instance. They are nowhere near being competitive anytime soon and are on pace to have one of the worst seasons in MLB history.