That's the theory. Sometimes the practice isn't so simple, when you have 32423432234 lines of spaghetti code, all tangled up. Surely twitter isn't that bad, but I've been in situations where someone cited that very idea and it just wasn't feasible.
Unless you have massive engineering resources that's not dedicated to feature enhancements and new developments of your main product (ie. a small startup environment), a full rewrite is just too expensive.
