More to the point if you’re starting a new company you have to plan for success. Success means hiring more programmers and picking a language that’s fallen out of favor will inhibit every programmer who comes after your co-founders. Assuming you’ll be hiring junior programmers it makes a lot more sense for your experienced co-founders to learn something new then trying to teach your junior programmers something old.
(Look at any programming language that's fallen out of favor in the past and you'll see the salary of coders in that language goes up as the language's popularity goes down)
The problem you have is you, as an outsider, can see their problem but you don’t have the knowledge or experience to suggest a solution (“cooler than php” is not a good argument). If I were you I’d seek out PHP programmers who made the switch to either Rails or Python. Invite them to a lunch with your co-founders and let them talk through the issues. PHP has some significant advantages over Ruby and Python yet people are still gravitating away from it. Getting them in the room with people who are part of that movement is your best bet at convincing them