I put a massive disclaimer on my viewpoint, as quite frankly I haven't followed properly since 2005/06, and tbh, if Bret Hart's return didn't get me to watch it again properly, then nothing will I think.
So I could be a guy who isn't familiar enough with the current state of the WWE to properly comment. What I can say is that I wholeheartedly think that the split titles played a part in holding the title meaning slightly less than it does a few years ago.
Anyway, it seems like in terms of "big names/flagship wrestlers", Raw always has more of them. Which makes the SD title scene less "reputable", even if it does have the midcarders who are awesome in the ring and people want to see get a push. Putting gold on them doesn't automatically mean they've got the cred or it has the meaning that it did when Foley won the title for the first time.
One of, if not my favourite wrestling moment ever is still Chris Benoit (gasp, I know) winning the title at Mania 20, and that was when the titles were split, so I don't think it's the worst thing ever or anything. But still, being WWE World Champion was just so much better than what is realistically a Raw Champion and Smackdown Champion, no matter the random differences they give the names of the titles.
I do agree with more or less everything you say here, I do think having two titles devalues the prestige of the achievement somewhat, and I really do miss the days where it was possible to identify one guy who was the absolute top of the company. However, I think it has got to the stage where that aspect of the WWE is just a thing of the past. As GIMH has mentioned, to have one world title at this stage would just not work given the way the Rosters are broken down and how the company generally operates these days. Having one world title just wouldn't really be feasible with the brand split still intact, and getting rid of the brand split would massively limit the chances of the developing talent. It's worth bearing in mind that the general strength of the roster is tenfold what it was back in 2002, or even when the split was introduced. Back in those days the undercard of a PPV would often include dross that nobody much cared about, like the Holly's or the APA vs Test and Albert or something equally mundane. That sort of undercard just does not exist anymore, in comparison to the smaller PPVs of the early 2000s most of the PPVs these days look like super cards. Though I've never been a huge fan of teh brand split for most of its lifespan, I would say that it really is crucial in the development of younger talent, and for it to work properly imo it needs a prestigious title. Having a world champion appearing on both shows would complicate things uneccesarily imo, having the World Champion enter into a feud with someone on Smackdown but then show up on RAW every week would not only be pointless, but would take attention away from whatever else RAW was pushing as it's big storyline. Similarly, ending the brand split would result in Cena, Orton, Edge, HHH, Taker, Rey, Big Show, Nexus, Miz, Kane, Punk, Truth, McIntyre, Morrison, Kofi, Christian, Swagger etc... all in one place, and although making up a strong line up, would just clutter everything. None of the younger guys would get the airtime that they deserved, and those that I haven't mentioned would have their chances restricted massively. WWE has done well to shake off the image of not pushing younger guys over the last 2-3 years, and it would be an awful move to go back on all of the good work done so far.