Imagine you had airplanes be built the way they wanted, crashing from time to time, not starting and having people work on the wings to fix things in flight.
If this was something done for fun and without impact on people then nobody would care.
Suddenly, a Monday morning, someone says "woah, this cannot be - you have to fix this". But this is not fixable, you have to build a new plane from scratch, or completely review the existing ones. Planes would be grounded.
Now a software company: typically your old plane flying by more or less miracle (when it flies). You cannot fix it, you have to rebuild it. Either you ground the company and force them to build something new, or you will always have legacy.
The legacy is not fixable - it simply is not. You need money to redo everything and if you do not have the proper pressure then it will not happen.
Then, building a new company/software can be done the right way. This is not even difficult, I would even say that having these constraints will help in the overall quality. But this is a new software, not a "fix" of the old one.