I've had somewhat similar, if less cruel thoughts.
I think his voice is certainly still relevant to many, but it is a good first step as I see it. But a lot of folks already get that take on things, but have already moved down the road to the next step beyond that, and the step beyond that. The reality of it is, what seems revelatory to us today, which it is, becomes 2nd nature to the next generation. They're like, "So?"
One day the bleeding edge thoughts will seem old-school too, and we'll be seen as fossils arguing the obvious.
But you have to remember the "so what?" response is only possible because others who did the hard work made what's 2nd nature to them now possible in the first place. They aren't the ones who plowed the hard earth as they feast on the harvest it brought to them.
But Spong's points of view still speak to large segments grappling with those earlier stages of shedding mythic-literal dogma in favor of a more liberal, larger picture that includes others outside their own comfort zones. Just like at some point I'll be talking my 'old-school' stuff to those still working on sorting stuff out in that part of the conversation. Hopefully, I'll keep myself going on this trajectory rather than just start repeating the same old stuff. But again, that still does have its place and relevance. A constant drip of water wears a groove in stone.