Here's a deep breakdown by the Seattle bike blog:
“With fewer financial resources available, SDOT will focus on capital project delivery for existing work and commitments made in the levy,” the budget overview notes. “Less will be spent on maintenance and preservation of assets (roads, bridges, transit, pedestrian and bike facilities), while innovations and system enhancements will be delayed to a future time when more resources are available. This slowing of maintenance and asset preservation work will affect transportation safety, mobility of goods and services, and climate and environmental goals.”
The expiring levy has provided $103 million per year, and the only way to craft a budget without that $103 million is to slash pretty much everything.
TL;DR:
Without the levy, the current miniscule budget for fixes/improvements will be cut in half. Every major policy promise will basically die, and the bare minimum will be way more bare.