I don't think that's true.
Your 800x600 pixel image will take up 1/3 of the screen on your current MacBook Pro, and would also take up 1/3 of the screen on your Retina MacBook Pro.
Your 2880x1800 pixel screen would think it's a 1440x900 pixel display. It would just be sharper than a 1440x900 display. All apps would think they are running on a 1440x900 pixel display. It will technically be called a 1440x900 HiDPI mode. For Apps "in the know", they can compensate with double-rez images. For everything else, they will act like they do on a regular 1440x900 screen.
Read this for details:
https://www.macrumors.com/2011/07/25/os-x-lions-hidpi-modes-lay-groundwork-for-retina-monitors/
edit: presumably you could run it at a true 2880x1800 rez, in which case everything would be tiny.
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