Second, the "developer who started that" were the founders. One had worked at Microsoft, and so was very familiar with ASP.
Third, FogBugz was a complex, mature application by the time Django or Rails were even considerations for production systems. Phasing in in parts is not really a viable solution either. There was a lot of business logic written and shared that those new parts would need access to, which would create more work to implement.
In short, no, you did not. You're looking at decisions made 10 years ago through today's lens, without considering the state of the art at the time.
(In case you're wondering, I worked at Fog Creek from 2005-2012, though I was never a part of the FogBugz team.)
I'm not a huge PHP fan, but it would have saved 10 years of development and maintenance over their NIH solution. See the "Jumped the Shark" piece from Coding Horror... not a minority opinion.
It seems archaic now but those were archaic times.
They could have used Thistle to move to PHP ten years ago and saved a lot of money.
Then instead of cutting their losses, they doubled and tripled-down on it until they had their own language and sophisticated tools around it. Around this time, Django and Rails had been started already. And several decent cross-platform web frameworks were years old, such as CherryPy. Even PHP would have been a better choice. One of these could have been phased-in parts at a time to minimize disruption.
Did I get this right? Because there are so many WTFs that I must have missed something.