I don't know if you're entirely correct on the old school/new school comparison. It's not as if Google decided way back in the late 90's that they'd (re-)implement Writely in Java just because it was en vogue. That's a newer technology decision than implementing reddit in python...
Ultimately, the question of what to code in is not nearly as crucial to success as we're making it out to be. And answering that question "correctly" has a lot to do with who's coding, how the code's used now, how you think it'll be used later, and many other questions.
I think it's at least worthwhile to know why other people use boring languages. Then feel free to use cool ones instead, not because you're impressed with how well Basecamp runs, but because you know you don't need Google-sized scalability right out of the box.
I suspect Google decided long ago to create a server infrastructure for programs written in certain languages, which in turn dictated how they rewrote Writely years later.
Choice of programming language may improve performance by a constant factor at best. I can't see how Google-sized scalability depends on that. I'd be happy to grow 1/10 or 1/100 of Google!
Ultimately, the question of what to code in is not nearly as crucial to success as we're making it out to be. And answering that question "correctly" has a lot to do with who's coding, how the code's used now, how you think it'll be used later, and many other questions.
I think it's at least worthwhile to know why other people use boring languages. Then feel free to use cool ones instead, not because you're impressed with how well Basecamp runs, but because you know you don't need Google-sized scalability right out of the box.