People always say this (and I agree it makes sense), but it rarely works out that way. How often does any player get moved for that kind of package? The best example I can think of was Deron Williams. Sure, Chris Paul was traded for Eric Gordon, Aminu and what turned out to be the 10th-overall pick in the draft, but all in all, that hasn't really amounted to much for New Orleans so far. And those were trades for who were widely considered to be the top two point guards in the league. I love Aldridge, but he's not going to get the same haul that Howard got. And Howard himself only got a couple of decent, but not great young players (I do love what Vulcevic is doing right now, though), a couple of overpaid vets and three low-level first-rounders. Of course Howard was a cancerous player coming off an injury, but all and all, I think he was more attractive than Aldridge is.
The Spurs may not be able to provide a good enough package (especially by themselves) but someone will be able to. If Aldridge does get traded, I think the haul is going to be worse than a lot of people expect it to be.