Why lower rewards? Just keep the rewards for PvE the same as they are now
Because as I said there are cards and weapons which are better at PVP which you would normally want to bring in for regular missions on the off chance that an antag shows up, just like there are characters that are way better at dealing with PVE than dealing with an antag.
Like honestly give me a good reason to for example pick Harec over other raiders if PVP was optional, and no "thinking he's fun" doesn't count. His damage to objectives is worse than raiders like Schneider, Valeria and Hans and since his headshots kill elites rather than put them in a downed state there are much safer bets.
And finally because Mercurysteam is a business and current gold rewards like them or not, are balanced around the fact that you're going to lose matches or get a low score, thanks to antags showing up. You dont honestly think ANY company with a f2p model would just leave those values the same do you?
I mean why do you think the gold rewards for solo missions are a one-time thing? Because otherwise the most efficient way to grind gold would be to run missions solo, no bad players or antags to mess up your game and you can bring a character custom built to speed run that one specific map.
Blueprint missions, faction points (not everyone has multiple characters unlocked per faction), testing cards, trying a new weapon, using treasure/bounty hunter, a preferred character is locked, just for fun, etc. are all reasons why players would select suboptimal characters for PvE.
Also, there are plenty of gold sinks that could be added to the game. Converting gold into faction points, paying for experience boosters (technically already in the game but not worth it), buying loadout slots with gold (highly requested), rerolling weapon stats without upgrading, etc. You make it sound like players would be sitting on millions of gold when that is just an indication of not having enough gold sinks in the game.