Look at Sage accounts - if you dare! Everyone uses it, not because it's any good, but because everyone's accountant, suppliers and customers use it. A bigger heap of disjointed, buggy, flogged-to-death-but-still-bad and crappy code I could not conceive. It's HUGE. And it's based on very old code from the 80s, with bits added on over the decades. The only reason it works is because there's Sage maintenance available, and almost everybody uses it. Basically Sage software keeps working because there are dozens or hundreds of support staff helping the users when the database goes wrong every other month.
We support several small companies who use it, and it's their biggest problem. The updates never work first time for example. We support accountants who use it to support their customers: they spent literally tens of hours a month on the phone to Sage support.
There's no incentive to make the code work better, because they make plenty of money from support contracts, and people keep buying the new version every year.