Here's another controversial (?) opinion. I started typing this post in the Opeth thread (we were discussing diversity) but it definitely belongs here.
I don't think DT's lastest few albums have been very diverse. SC and BC&SL were similar to each other. ADTOE and DT12 basically for me were just representations of the core DT sound, albeit two different sides of it. They didn't introduce anything new to me, except for the orchestral section on IT, Beneath the Surface and the intro to BITS.
If you start at WDADU and go forward in time, there's a huge variety of styles in DT's albums. I&W is a more refined, mature and proggy version of WDADU, where the band found the sound they were looking for and all the tiny flaws in songwriting were fixed. Awake is structured differently, is a lot darker and isn't standard prog at all IMO.
FII was a lot more simple and maybe commercial, and while not everyone likes it and most people don't like a few songs on it, the good songs on that album are truly great and add diversity to DT's catalogue.
SFAM was the band's first full-fledged concept album and it sounded more 'modern', while also in a way paying homage to classic prog. More diversity. SDOIT, IMO, is their most experimental album to date by a mile, and while being classic DT (especially the second disk), it discovers new ground while still being amazing. Heck, it's my favourite DT album.
ToT was like Awake in a way, because it was a lot darker and heavier than usual. However, it sounded a lot different from Awake; for example, while Awake had a very distinctive feature in JLB's epic vocals like I&W, ToT had some really good melodies but JLB sounds totally different. Of course, this is logical regarding the vocal chords incident, but it still makes a large difference. ToT is heavier than Awake, has fewer songs and different structures. Not all people like ToT, but again, it IS different.
Octavarium was basically a summation of most of the things DT had done before, with the modern sound of ToT and SFAM. In my opinion, that's also diversity. Why? Because, for example, the heavy songs (i.e. TROAE and PA) are not as dark and thus couldn't have really fitted on ToT. The shorter, more poppy songs, like IWBY and TALW, couldn't have fitted on FII either. The album is a summation of styles, but the songs fit together really well and all have some distinctive quality which makes them a bit different.
Now we arrive at SC. For me, SC was a bit of a mix of Octavarium and ToT. It was a heavy and dark album, but not as dark as ToT. Still, here's where the diversity begins to decline a bit for me. There are definitely some new ideas on this album (PoW, Repentance, CM) but not all of them work so well, and the overall sound of the album isn't that different from things they've done before. Still, I think the album is diverse and differs in some way from the previous albums, and I like the album.
BC&SL marked another slight decline in diversity. Again, it was pretty heavy and dark, but now the album was focused on long songs and lots of instrumental stuff. There aren't a lot of new ideas/concepts on this disk, but there are some: the volume swells in TCOT and MP's more prominent background vocals (which also were more prominent on SC, and which I think most people don't really like that much). Again, I like the album, but it just isn't that diverse as an album and it doesn't offer a very great deal of 'new' sounds or styles.
ADTOE for me marked a big decline in diversity. For me, the album basically marks a return to DT's familiar sound without sounding very dark or very heavy. Still, it absolutely sounds like DT. I don't really mind this album not being different; it is a good album, and hey, you have to stabilize a bit after you've just had a founding member leave, right?
However, I was very much disappointed when DT12 came out. For me, while it would seem like the album is very different from the rest of the discography, it is not. Yes, the songs are shorter, and more focused on choruses. Yes, there is a huge orchestral section in the epic. Yes, there is a sort of Easter Egg outro to that same epic. But still, in my opinion its sound isn't different from the rest of DT's discography at all. In my opinion, this is basically just another summation of DT's sound which started with Octavarium. SC, BC&SL, ADTOE and DT12 do not sound very different at all; maybe ADTOE is the odd one out here, but that's only because it's less dark and heavy than the rest.
Someone who had attended the listening parties for DT12, I don't remember who it was, said on this forum that DT12 sounded a bit like SC and BC&SL style mixed with I&W and Awake's shorter songs. That got me very excited. I love all of these albums. And in a way, it did sound like that. But it didn't sound like that in a way that I had expected, or rather, in a way that I hadn't expected. What do I mean by that? Well, I had expected something new, which still retained the DT core sound. In a way, I had expected DT12 to do something similar to what SDOIT had done, which was experimenting with new ideas while retaining the core sound. And with a new drummer, I had expected something new. Well, for me it wasn't.
While I agree on your general statement, I think the big decline in diversity started with SFAM onwards. Yes, there are changes, but nowhere near the differences you find on the I&W-Awake-FII run, the most diverse in the entire catalog. In Scenes, which is a back-to-I&W style with the fresh view of Rudess, they seem to have found their sound, and started to work everything from there. By the way, both Awake and FII are more modern than Scenes, or any DT album, not that it's necessarily a good or bad thing.
Going back to the changes, they started to change less and less on every album to the pinnacle being BC&SL and ADTOE for different reasons.
DT12 is an album where they achieved something they didn't achieve since Awake: writing shorter songs without sacrificing their progresive style. Every other time DT did that, was at the expense of their proggier side, most of the times it was short = straightforward(again, not that it is bad thing). Sure, no big changes here, but for me, the last big change was FII, the last surprising but not that big one was 6DOIT. All the others, while they incorporate new things to DT, they do it within the framework, so to speak, so I don't agree with ADTOE being a decline in the diversity department, and nor is DT12.
And by now, considering everything I said, I think that we won't see huge changes anymore, it's a common thing as band members get older, they find their zone and, considering how wild DT fans can get when they do change something(every new feature had screaming haters ever since I&W), I don't think they have a good reason to, unfortunately.