P: Merge Photoshop, Illustrator, and InDesign into one application

Well, yes, but there are people who use all of them. Plus I don't see that much of difference 🙂 I guess it would be bad if they would have a lot of difference 🙂

From version to version these software become more and more similar, it is like on its way to become one.

"have UI tailored to different customers"
Could you please bring an example of what exactly could be a critical problem?

"Combining them would make a much, much bigger mess."
I think "much bigger mess" is now 🙂 all these plugins, features absence causing jumping from one software to another, and then back, all these problems with "import/export".

You guys already hiding a lot of stuff under "show options" button in the widgets, why not to add few more settings in there, and you have a tool bar and a right click menu with additional settings. Photoshop's right click menu such a useless thing btw, if you would add "search" in the layer pallet that menu could be easily replaced with something else.. an atavism, really. And Illustrator doesn't have a right click menu in the layers pallet at all (interesting why) :)

And now I'm thinking, "at least" should be not Photoshop and Illustrator, it should be Illustrator and inDesign 🙂

The serve very different functions:

Photoshop: Pixel image editing

Illustrator: Vector graphic editing

InDesign: Page layout

They work together, but they do very different jobs. It's like asking the Butcher, the Baker, and the Candlestick Maker to be one guy.

All 3 of those applications work very differently, have UI tailored to different customers, have large numbers of legacy documents to work with, etc.

Combining them would make a much, much bigger mess.