Rewriting any system does not take much money, it takes a lot of time and a pair of good devoted hackers. If you stuck with bad ones, even money won't help. I believe, twitter's hackers are good enough to deal with the problem and I don't think their system is so complex. It's too young to get covered with all the dust and rust :-).
- Re-writing their whole system incrementally will be expensive; I think they might need google money and brains to complete the whole thing... or a lot more than 15mm.