Can't I just adjust contrast on my 2013 mbpr by downloading an app or something.........
No, no, no, mate
I recommend you read some Display reviews and tests of laptop displays on
www.anandtech.com to get a better understanding of all of this.
There's a difference between the displayed contrast - i.e. what you're talking about, and the range of contrasts the display can show at once with granularity. Think of it this way. A display with a higher contrast ratio can display many more shades of each colour, where as bumping up the contrast merely increases the difference between the colour values.
The new screen will still look better with any content though the difference will be limited to the bump in contrast and brightness, unless you view content specifically made for the P3 colour space. macOS is the only colour managed platform aside from iOS, meaning it'll automatically show the right colours no matter if it's sRGB or P3. If you however plug a P3 monitor into a Windows PC, what it'll do, is actually show incorrect colours most the time, until you show an image made for the P3 colour space. Microsoft's own Surface Studio negates this issue by implementing manual user switching of colour spaces, so the user has to know which colour space the content is in, and change the entire display's colour profile through a LUT (look up table for converting colour values). macOS/iOS is brillant when it comes to this stuff and just deals with everything on its own.
Even if you get a 5k iMac, your image won't look sharp, if it's only 1MP. To get an image that takes up the entire screen and looks sharp as possible, it needs to match the display resolution or exceed it, at at least 15MP. Same thing goes for colour. If you shoot an image with a certain colour space, how would the image improve from being shown on a better screen? The problem outlined with Windows above, is that it can only assume content fits one colour space at a time, and either takes everything as sRGB or everything as P3 or whatever. Individual apps on Windows can colour manage themselves, but on macOS, it's within the OS.
You can play 32kbps audio on the best speakers in the world, and it'll still sound ****. Both content and output device need to match, otherwise one will bottleneck the other.