Being on the Switch, loading screens don't have to take long at all for a game like Animal Crossing. If there was something similar to main street, there shouldn't be too much needing to be loaded into memory (RAM). Probably a quick screen transition, and bam. Maybe 1 second.
The island is too small as is. Spreading shops all over the place would clutter it. Think about all those shops, 8 player homes, and 9+ villager homes. That's a lot of real estate.
EDIT: The 1 second wait is doubtful though. The 24-minute gameplay video shows a 5 second wait (not counting the animation) for entering the Nooks' tent
So I noticed in the E3 trailer that on the map their looks to be multiple tiers of land. I have a feeling that the lowest tier will be your land for housing and decorating and stuff, and I feel like the second tier will be where the towns buildings will pop up. I feel like they will unlock as you pay off your loan, cause I heard that as you pay off your loans the money gets reinvested into the community to improve things. There were two other tiers that were much tier than the first two and I believe those are just their as mountainous areas and have no purpose other than visual.