Silva is 32, if City signs Isco you can bet Guardiola would use Isco to keep Silva in reasonable minutes in the next seasons, and Isco would get to play LW, both CM roles and even as a False 9.
That would leave City with a front five for 8 players, Sane, Agüero, Sterling, Isco, Silva, KDB, Jesus and Bernardo. It might sound absurd but if you look at Real last year having Morata, Isco, James and Asensio as their deph players to rotate their front 5 of BBC, Kroos and Modric, or the rosters PSG and Barcelona are assembling (Di Maria, Draxler and Lucas warming the bench in PSG. Coutinho, Dembele, Iniesta, Messi, Suarez, Rakitic, Paulinho, Alcacer and maybe even Griezmann this summer for Barcelona) you can start to see a pattern.
If we thought the rosters some teams had years ago with 14/15 world class players were stupidly strong, right now the squads teams with money are assembling will put those rosters to shame, if you want to fight three titles with your club you're going to need that depth, and money won't be the only problem, you have to sell those players a winning team so they can come and say "ok, I'm not going to play 4k minutes per season, but it's going to be worth it".