About 2 months after the game comes out they make a small dlc and release it every Tuesday (a small 1 hour quest or new items and gear ect.) The two months would give them time to plan out the first few. A reasonable charge would be $1-$2.50 per dlc depending on the amount of content.
At the same time they have the larger team of Fallout 4 staff work on 2 or 3 very large dlcs to release over the course of 12 months+. (Ex shivering isles or tribunal size)
This would generate revenue; keep consumers interested, and make the wait for dlc more bearable. They can release graphical patches and support the game with bug fixes ect as well and include them periodically.
Why don't they?