It makes no sense for Lewis to go Ferrari. Lateral move with very small chance of success. They are in the process of a team overhaul and really won't be ready until 2025-2026 if you listen to Vasseur and people close to the team. Norris and Piastri aren't going anywhere especially given their age, relative speed, and the lengths McLaren went through to secure the latter. It would be very short-term thinking from Lewis to jump to McLaren just because of a midseason upgrade anyway. I would think he's much more thoughtful than that.
It's different jumping to a up and coming team when you're 28. When you're 38 looking for another title, that's not a challenge you're looking to take on.
It makes less sense for him to stay. You're kinda incorrect on the timeline. The timeline isn't just this mid-season. They haven't been listening to him for 3 seasons. The battle with Max for the title he wanted them to develop the car in season and Toto said no. This is the 2nd year of the new regulations and they're only in 2nd due to Ferrari flubbing strategy so much and being inconsistent. The Merc engine is the worst of the big three and can't be developed until 2026 (This is a pretty big political play by Red Bull to get an engine freeze). They don't seem to want to follow Lewis' lead going forward. Lewis is literally in interviews saying it. They're currently being caught up by one of their customer teams in McLaren. Williams car design is apparently better than what the Merc engine can handle.
The midseason upgrades are a sign for who clearly understand how to improve their car. It won't get worse from here on out with McLaren. It's been two completely different tracks in a row and they're cooking. It's not short sighted for a 7 time champion to decide to leave a team who hasn't listened to him, or George, on how to develop their car and they're clearly losing ground. My point was he needs to go to a team that takes his input into account. Ferrari would be that team. Their engine is already cooking. They just need someone to point them in the right direction developmentally and it ain't Carlos Sainz Jr. or LeClerc. Ferrari historically have built very strong cars when they have champions in the stable.
Edit: Does anyone else remember the rumors back in March that Lewis was going to sign his Mercedes deal in a "few weeks"? It's almost August and he hasn't signed a deal yet. I'm willing to bet that's for a reason.