The DS replaced the DI which I think was available for about 6 years, so the DS probably has some time to go yet. Plus they just came out with a new version of the DS (76 keys) less than a year ago, I doubt they'd have bothered if a replacement model was imminent.
DS88 does not have speakers. For a board with speakers and a large variety of sounds in the same price range, you could consider Casio PX-S3000, PX-360, or CGP-700. The new PX-S3000 has the best piano sounds of the three, and their newest action. The other two models are pretty similar to each other and have a nice touchscreen interface.
I was talking about the DGX' ability to display sheet music on its screen. But since you mention it, I don't think the DS88 has any attachable music stand, either. I guess you'd put a traditional floor-standing music stand behind it, or maybe someone has another idea.
88 feels much more like a piano. 76 and 61 are non-hammer action keys, not as good for piano, but better suited for some other sounds like organ, and also lighter weight for better portability.
Sounds are subjective... and you'll probably like some sounds better on each one. If there are some sounds that are particularly important to you, you could focus on comparing those.
I think it's market positioning. P125, DGX660, probably any model with speakers, is designed largely for use in the home. DS88 is more of a live performance board, where working with printed sheet music is not as common.
You can play piano on a non-hammer action keyboard, but dynamic expressiveness suffers, more so on some than on others, as it is much harder to control gradations from soft to loud. That's not always important, but sometimes it's very important/ Accuracy can also be more difficult, as adjacent keys to the ones you target go down more easily. Whether you need 88 keys depends on your repertoire and style. Though it's also useful for non-piano work, when splitting multiple sounds across the keyboard.
