For me, it's not an eyesight thing, because I have good eyesight. I'm an avid reader and don't use or own reading glasses. It's that the P12 characters are tiny and the panel is at a very slight angle - less so than the Poly Evolver Keyboard. Just compare their profiles.
The soft buttons get into an entirely different discussion related to the complexity of the instrument vs. the actual available parameters. I much prefer parameter-per-function, which, on the P12, would have resulted in a physically immense synthesizer. Dave's solution to this was to get the most out of the display.
Yes, the P12's window is physically larger than the PEK's, but the PEK has two windows. One displays only the program number in large characters, while the other gives parameter information or program names in no more than two lines. The P12 gives all of its information in one window and does so in as many as five lines. Hence, with a glance of the PEK you can easily see all the information, but with the P12 you need to come in close, because the characters are so small, the information so detailed, and the panel almost flat. This is greatly improved, of course, when you're done programming and have saved your sound. Then the P12 display reverts to a much larger character size and shows only the program number and name. But once you again touch a parameter, you're back to tiny characters.
I disagree with your assessment of the displays.
The P12 OLED display is MUCH sharper and clearer than the 16x2 LCD found on the PEK.
This allows for fonts and information to be smaller, yet still be clear and easy to read.
As 'BobTheDog' has already explained, the use of soft buttons necessitates that more information needs to be displayed for certain tasks on the P12 compared to the PEK.
However these parameters are only displayed whilst actually editing anyway.
Once you're playing you can have both the patch name and program number displayed in nice large text, or as (is the case mostly for me), not at all as I have the screen saver option enabled.
I would concede that it looks like DSI could have perhaps offered the option of a larger font in certain areas.
For example on the 'Low Pass Filter' menu it has the Header Menu at the top... LP Freq| Resonance| 4 Pole | KeyLp Freq |and then it displays the rotary dial position and the values underneath, and then again underneath the dials and values it has the labels of LP Freq| Resonance| 4 Pole | KeyLp Freq | repeated.
You really only need that info once, so yes it could be argued that you could lose one and use the space saved to have a bigger font or even just display purely numerical values.
I for one are pleased that they didn't and here's why...
The smaller fonts are pin sharp, easy to read and because of the clear space between the elements the interface is clean and uncluttered.
Also it keeps the UI consistent.
For example in the 'Assign Mod Source' window, the Header Menus at the top clearly define what each column below it represents, which makes using it MUCH easier.
It makes sense therefore from a UI point of view to have a consistent interface with a Header Menu for every parameter.
The display of the rotary encoder as a graphic on the display also means you have a visual representation of where the knob is - this is helpful in a poorly lit environment when you perhaps cannot see the line on the knob itself and is information that isn't conveyed as clearly by just numerical values alone.
Having a header on some selections and not on others, or big fonts on some menus but not on others would lead to an inconsistent UI a poor end user experience IMO.
Please tell me exactly what you have to 'dial in' that needs to be to the digit accurate yet also done on the fly?
It'd be interesting to know as I honestly can't think of one myself - but then I don't play two synths at the same time with one hand either - I'm just not that good a player!