As much as I don't want to sell him to a rival, I also don't want him to walk away to a rival for nothing.
At least if he goes to Man City, we get 50m for him.
If he goes on a free, he could go to Chelsea or Man U and we could see them winning the PL title with him.
It's elite players like Sanchez that win games on their own and it's the reason the big clubs buy these players.
It's also the reason that clubs who don't have these world class players tend to struggle.
Like you, I'm not sure that Lacazette or Lemar are going to be good enough to make up for him leaving.
The only way round it is to either get him to sign a new contract, or buy a genuine world class player.
I wonder if Sanchez would have signed a new contract if Wenger had been a bit more ambitious in the market a couple of seasons ago, when he bough just a GK, rather than waiting until he's desperate to get a couple of decent players.