A good part of their lethality was the fact that early war Air Defenses ranged from "merely" bad to out right pathetic/nonexistent. This is both defending fighters and AA.
Bad according to what standard? That is an absurd statement. First, you seem to be evaluating 1930s and early 1940s aircraft by some kind of notion of ideal 1944 or 1945 or post-war anti-aircraft defense (and fighters). Which makes no sense. That's like saying a Sopwith Camel sucked because an F-16 can outrun it. Second, you are assuming that these aircraft stopped being effective as soon as there were 'good' fighters and AAA around, which just isn't true.
The key difference between success and failure with the SBD was very clearly training, as you can see the marked difference in outcomes between US Navy SBD units vs. Marine and Army units.
But SBD were still sinking ships in the Battle of the Philippine Sea in 1944, by which time yes even the IJN certainly had good air defenses. Meanwhile, in the same Theaters at the same times, multiple other aircraft, some which much heavier bomb loads, had
far less impressive or significant impacts on enemy forces. This was also measured in Navy testing and post-engagement evaluations, in which they determined that the SBD has a far greater accuracy than their fighter bombers, and was needless to say lightyears ahead of the heavy bombers and other level-bombers, unless used with mast-head height bombing etc..
It did, however, become vulnerable to enemy fighters by the mid-war. The problem was the replacement (SB2C) was barely functional, and the British equivalent (Barracuda) even worse. So they continued to operate SBDs. At the Philippines sea they sent 26 of them on that infamous strike in which so many aircrews ended up in the drink. All the SBDs made it back.
The Ju 87 in particular made a lot of it's reputation over two years before the SBD got into combat.
That's a gross exaggeration. Yes the Stuka became somewhat (in)famous during the Spanish Civil War and the Battle of France, but the Ju 87 continued having major successes in Russia and North Africa. It inally began to be phased out in the West and Med in 1943. It was devastating during Operation Pedestal and was used at Salerno, and quite tellingly at Kasserine pass (in Feb 1943). The British destroyer Panther was sunk by Ju-87s on 9 October 1943.
They continued to use the Ju 87 successfully in Russia into 1945, they were used during the defense of Berlin.
In fact I'd say, the incorrect use of the Ju 87 at an (attempted) strategic bomber in the Battle of Britain gave it a kind of premature bad reputation, which it more than redeemed to the dismay of the British years later in North Africa.