Probably need to chicken-egg how to load/unload cargo to what first. And unfortunately, the need to create an entire system is going to severely inhibit development.
For example, if I have a viable method and means to deliver cargo from seaplane to ship, a seaplane would be great for COD-type missions, and you could bypass the carrier and supply a SAG with large amounts of cargo. A bit like the V-22 plan, but able to deliver much more cargo, over much further distances, and probably faster.
BUT, since there isn't such a method that already exists, there's not much motivation to invent one because I don't have any seaplanes. And if the Navy needs to build seaplane tenders again to make it viable, it would be an uphill battle.
And there isn't much use for developing a cargo seaplane for SAG delivery, because I don't have an extant ability to use it.
Best use would still be SAR/MPA/ASW. You can add strike capabilities, but what's the real benefit over land-based strike aircraft? I can disperse them, but they still need a logistics train for operations which will include fixed facilities.
I could see some very interesting EABO tie-ins IF you simultaneously develop a method to get from plane to shore. Say you develop a small landing craft (and a system to load and launch it) that fits in the seaplane so you can launch a preloaded LC directly from plane to beach without facilities. But again, we'll now need to develop an entire family of systems to make the seaplane viable. That's big money.
A SAG centered on seaplane tenders able to support large seaplanes for cargo, ASW, AEW, and strike is interesting to me. Might ultimately be more utility than a CVBG and cheaper, but it doesn't have any interceptors or CAP outside a landbased tactical air umbrella. And good luck getting the Navy or Congress to go for it based on completely unproven napkin math. Seaplanes have a pretty high writeoff rate on the open ocean. Do I really want my expensive AEW assets to be landing regularly in the open ocean to support my SAG? Probably not.