Imagine you're working on this or that feature, and find a stumbling block in the legacy codebase (e.g., a poorly thought out error handling strategy causing your small feature to have ripple effects you have to handle everywhere). IME, it's literally cheaper to fix the stumbling block and then implement the feature, especially when you factor in debugging down the line once some aspect of the kludgy alternative rears its ugly head. You're touching ten thousand lines of code anyway; you might as well choose do it as a one-off cost instead of every time you have to modify that part of the system.
That's triply true if you get to delete a bunch of code in the process. The whole "problem" is that there exists code with undesirable properties, and if you can remove that problem then velocity will improve substantially. Just do it Ship of Theseus style, fixing the thing that would make your life easier before you build each feature. Time-accounting-wise, the business will just see you shipping features at the target rate, and your coworkers (and ideally a technical manager) will see the long-term value of your contributions.