There's a difference between being a scapegoat and just being ****. De Ridder and Martin are just ****. It's not scapegoating, I won't boo them when they come on, I won't ironically cheer when they do something well, they're just **** at football. Relatively, before some smart arse tries the "you're no better!" retort. (I probably edge out De Ridder)
You can try to rationalise it all you like, but the treatment they get is out of order regardless of their ability. Your opinion is that De Ridder and Martin aren't good enough, other people think that about Boruc and Rodriguez. It all seems very subjective to me. No-one ever says Jack Stephens or Ben Reeves are **** but they're clearly not good enough for the first-team either (yet).
No-one is suggesting Martin should be first choice, but if somehow he gets picked he needs to be given support like any other young player breaking into the side. Imagine if Shaw had been booed when breaking into the team, I don't imagine it would have helped his development.
Every player at the club needs support, and frankly someone like De Ridder could run past 11 players and backheel the ball into the net and he'd still be described as hopeless.