Yes, the cost of change by definition increases with the complexity. I don't think that is in contention. Why is it changing for any other reason that you're growing (or trying to stave off decline?) For internal tools, change may be a function of external business pressures (like a supplier going out of business, requiring changes in an internal tool), but that is asking for new software.

As you add libraries, the cost of maintenance increases because the surface area of security increases. However, short of major changes (React, Rails, Etc), this feels like it's not moving outside the 10-20% range.

Then, there's the cost of keeping it on.