Let's cross this bridge (and roads) - open to debate -

How to make them viable without affecting gameplay.
Add natural disasters like storms and huge waves to make bridges less reliable than terraforming, costlier to maintain but faster to deploy for temporary operations. Storms damage and accelerate degradation, requiring frequent maintenance and causing resource spikes when repairs are needed.
Bridges use many resources for upkeep, and each truck crossing increases wear. Poorly maintained bridges collapse. Storms can cause partial or total damage; partially damaged bridges risk collapse. Bridges provide quick access to distant areas early on but require Tier 2 mid-game to almost match terraforming. Tier 3 enables reaching farthest points without teraform the entire map.
T1: Tier 1 trucks (10 steel + 20 bricks/sq, maint. x1)
T2: Tier 2 trucks & excavators (30 steel + 40 bricks/sq, maint. x2)
T3: All trucks, excavators & trains (60 steel + 90 bricks/sq, maint. x5)
Pros: enable sea crossings, fast paths
Cons: degradation, expensive,damage

The goal is to make bridges valuable in the endgame when resources are abundant. This way, you won’t need to terraform every island and can reach distant areas using trains over bridges, accepting maintenance costs for long-distance operations. Terraforming remains meaningful for mining, crossing mountains, and flattening terrain for construction.

sure - let's complicate the game so much that no one will want to play it because of how hard it is to balance, maintain, find and fix issues, too much like a city builder rather then an automation game (roads, anyone ?) more about the people management then production lines...
just go play any of the other city builders or settlement games out there - there are quite a few broken like Banished, Foundation, City skylines 2 etc...

"Balance, maintain, find and fix issues", you are describing the actual game. I thought this was a suggestion board. Telling others to go play other games doesn’t seem very kind, and there are better ways to disagree with an idea—maybe by improving it or clearly explaining the problems. The developers already said they are planning roads; maybe they won’t release them, but would you really go play another game if they do?