I've replied to what I think is the substance of your post - the point regarding costs - in another reply below. Just for completeness, a couple of brief responses in relation to your other points:
Bullshit. The aim of these policies is to pad prosecutor conviction rates so they can appear "tough on crime". No prosecutor ever got reappointed, or reelected, because he released innocent people.
Personally, I think that elected prosecutors are a very poor choice, and I agree with you that they lead to over-zealous prosecutions (especially where the defendant has a high profile).
Sentencing discounts, however, are not necessarily related - they are offered in jurisdictions such as the UK, that don't have elected prosecutors.
I don't know whether elected prosecutors actually try to take advantage of plea bargaining in this way - I suspect that there's a countervailing influence because they want a big showy trial - but it might be the case. If they are, however, I think the solution is to get rid of elected prosecutors.
This much I agree with you on. The system wants to deprive us of our right to a trial.
There will always be innocent people who are convicted - either after a guilty plea or without. I'm not arguing that we should therefore be complacent, but I think that there are safeguards that can mitigate the additional risk to a satisfactory level - four reasonably common examples would be i) the right to see a lawyer for free; ii) judicial scrutiny of confessions; iii) a requirement for confessions to be in writing; and iv) the ability to retract a confession (the confession would then be evidence in the trial, but could be rebutted if, for example, there was evidence of coercion). Those are all fairly common, and, in my view, with all four safeguards the additional risk is reduced sufficiently for the trade-off to be a positive one.
Remember that (as detailed in my post below) removing incentives to plead guilty has negative consequences too, so there is a balance to be struck.