The sheer size of the thing is a big problem. Getting ~100KLOC of C++ compiling on a "new" compiler has been something that's taken me a few days before. Not

The sheer size of the thing is a big problem. Getting ~100KLOC of C++ compiling on a "new" compiler has been something that's taken me a few days before. Not too unreasonable. 100MLOC is literally 1000x the size - which extrapolating from "a few days" would mean it'd take me 8-14 years to fix up on my own were I to do it by hand. Or more realistically, the pace of change/growth on a codebase of that size probably outstrips my ability to convert it. So now even a simple straightforward task such as fixing up technically nonstandard C++ syntax to standard C++ that multiple compilers will happily accept, is a task requiring either automated code modifications (scary!) or a large coordinated team effort. If 100 people working together are, optimistically, 100x as efficient as me alone - i.e. there's no overhead in terms of coordination and planning - it will still take them around a month to do the job via brute force.