1. Vertical feed still exists, we call it a "walking foot" now. Most use an attachment rather than full time. Some upper end machines have walking feet still.
2. If you look into the head of a Davis VF it is like a Swiss watch compared to a Singer of the same period. Complex & expensive. more skills required to service as well.
3. This machine was introduced during the heyday of the industrial revolution when the goals were to invent a new and marketable thing but very quickly standardize to take advantage of mass production. Accessories for this are not like accessories for other machines.
4. people fear what is new...