Prediction: This show WILL have a downer/depressing ending.
Stan Edgar foreshadowed it. The Boys can't defeat the greater system of money and corporate greed. They will not stop Superheroes from being produced. The fact that they are the main forces fighting back against evil Supers and yet they themselves are borderline incompetent at it (Seems like Homelander has killed more supes than The Boys), they can hardly get definitive victories, they often create collateral damage, the system keeps trucking along despite their actions, they constantly fall out, etc.
The fact that The Boys: Mexico is set to take place AFTER the main series means that things will be more or less business as usual after the series wraps up; Compound V or an adjacent product will still be around, and so will a market for supes and the government tolerating them.
There's likely going to be another big-bad Supe to take Homelander's place... maybe Ryan?
He has Homelander's power (and more), he's capable of disregarding human life, he has a childhood built on trauma, etc. I definitely don't see him switching up and becoming the purehearted Superman of The Boys world.
I think Homelander will die and the public will mourn him, Butcher certainly will die without having accomplished much beyond his feud with Homelander; MM, Frenchie, and Kimiko will very likely die, ending the latter two's hopes of starting a family; while Hughie and Starlight (who likely loses her powers) will likely have to move on and put all of this behind them while Vought rebrands and lives on with new supers to replace The Seven.