I understand the constitutional point you're making, but I think we're conflating two things: exercising your right to trial, and showing remorse for what you did.

The right to trial isn't being penalised. You get a fair trial either way. What's being rewarded is accepting responsibility and saving the court's time. That's not the same as punishing you for exercising a right.

I'll grant that when the sentencing gap is extreme, the distinction becomes academic. If you're facing 20 years at trial versus 2 for pleading, then functionally you're being coerced regardless of the theoretical justification.

But in principle, rewarding people who show remorse is part of justice. Someone who accepts what they did and shows contrition is different from someone who forces the state to prove its case. Both have the right to trial, but treating them differently at sentencing isn't inherently unjust.

The question is whether the gap has become so large that it's effectively coercive. That's an empirical question about how plea bargains operate in practice, not a constitutional one about whether they can exist at all, which, if I understood it right, is your position.