I can't make sense of your reasoning OP. Let me know if I've misstated something
1) Silver is too easy
2) The hardest thing about it is all the bad players there (i.e., people playing silver are so bad that they actually make the game harder, instead of helping)
3) [Presumably because people find silver too easy] they try to play gold, and since they're really too bad even for silver, they screw up gold games too.
4) Therefore, make silver harder to keep the horrible players there from feeling like they are too good for silver, thus keeping them out of gold.
I can go along with 1 and 2 for the sake of argument, but you loose me at 3. If they players on silver are so bad that they actually make the game harder, how on earth are they coming to the conclusion that they're too good, and need to move up to gold? Maybe there is a slim category of people moving up from silver to gold that are generally top of the silver board, but still skilling up to Gold-ready. Making silver harder, and leaving gold's difficulty alone, would provide a better training ground for this very slim portion of the gaming community, but I don't think it'd solve the problem you're talking about.
And even if we accept 3, why would making silver harder -encourage- people to play silver? By increasing risk relative to reward, you make silver -less- desirable, not more. You'd have to make both silver and gold harder for that to work, but then why not just ask for a platinum difficulty where no noob would dare to tred.
To keep people on silver, you need to either make silver easier, or pay more credits. I think the source of your stated problem (and I'm not ready to agree that it's a problem), is that gold isn't -that- much harder than silver, but pays double or more the credits. Pair that with the fact that even getting past the wave 6 objective pays as much as silver, and you get a lot of people saying, "heck, why not"