When Brad took JB off the floor, he replaced JB with guys like Jae, Avery, and Smart, each of whom have the reputation as plus defenders.
CLE has no such luxury.
As far as getting targeted, GS was pretty much able to get any shot off at any time since Lue insisted on playing Korver, Deron Williams, and (to a lesser extent), Richard Jefferson. Maybe Lue would have played Jackson for some of Deron Williams' minutes (since for some reason he couldn't bring himself to play Derrick Williams, even though he was the only other guy on the roster who had the length and athleticism to give KD some problems and had shot 40% from 3P over the season).
And then there's that James guy, who while doing heroic work on offense was pretty much a turnstile defensively all Finals long.
LeBron's dirty little secret is that he's a pretty meh wing defender these days. I mean, he's always rated as a fairly average defender who has been able to turn it up a notch or three in the playoffs; but as he enters his mid-30s that switch will increasingly fail him. I suspect the first signs of age-related decline will be — and have been — more evident on the defensive end than on offense (where his combo of elite hoops IQ, court vision, passing, and handles with vastly improved shooting should age pretty well). If I'm the Cavs I'm already questioning how suited he is to chasing younger, quicker wing players out in space, and wondering when it will make sense to install him as basically a full-time 4 (assuming he even sticks around after this year).
As it stands, the Cavs are a defensive tire fire who over the last couple years — including this Summer — have pretty consistently addressed the problem by adding gasoline. First there was unplayably slow Channing Frye, then last year it was old man Korver and slowpoke Deron; this summer it's been an even older Korver, plus Calderon and Rose. You almost could not handpick a slower, more useless group of defenders. (As you note, Derrick Williams was an attempt to address the problem, but he couldn't find his way off the bench).
Flipping Kyrie for a guy like Bledsoe who can actually defend would no doubt help some. Adding a young, versatile athlete like Josh Jackson to the mix could help too, but as DeJesus notes, rookies are rarely defensive beasts; and one wonders how much Lue would trust him in crunch time, or if as a rookie he'd end up just a younger version of Derrick Williams.
The deal that could have really upgraded their D in a qualitative way (similar to the Warriors cutting loose Landry and Jack to sign Iguodala; or benching Lee for Draymond) was something revolving around Love-for-George — which would have allowed LeBron to fully embrace his defensive destiny as a 4, or even occasionally 5, and was apparently thisclose to a done deal before Kevin Pritchard pulled the plug. I guess we'll never know if David Griffin could have closed that deal had he not been cut. Oh well.