Sure, FogCreek thought it was a good idea at the time, but over the years it became a significant burden, to the point they had dedicated staff working on just keeping Wasabi alive. Time was even spent writing an internally distributed book about the caveats of the language!
I know you will dismiss this as "routine", but it's not...
For a small company, this is an enormous waste of time, money, and energy.
A big company like Google or Microsoft can afford to throw developers by the dozen at internal proprietary languages and not even blink -- but according to the article, FogCreek did blink every time they had to dedicate time to fixing it. It took time, money, and energy away from their core business - making software.
That's a lose condition.
FogCreek should have bit the bullet and re-wrote their application in an open, standardized cross-platform system. They would have been able to spend zero time worrying about the language, and 100% of their time worrying about their application. They could hire engineers off the street and have them produce in days-to-weeks instead of weeks-to-months. They would have saved themselves an enormous amount of time, money, and energy invested in a language that is now dead anyway.
It may have seemed like a good choice back when the decision was made, but in hindsight it appears to have been a very poor, short-sighted choice.