Most vintage cast iron pans were ground flat, they only stopped doing that as a cost saving measure later on.

My vintage flat cast iron pan from the 30's keeps its seasoning just as well as my modern one, and is a bit more non-stick compared to the modern ones.

What determines if a seasoning will flake off is mostly due to the type of oil used to create the seasoning. Flax seed oil will create a much harder seasoning, but it is the most prone to being chipped or flaking off.

Most other types of fat, like Crisco (don't cook with it!) or canola oil, will produce a perfectly good and resilient seasoning on smooth or bumpy cast iron.