I was about to say, "Why don't they just spray-wash the bridges a couple times each year, if it could double the lifespan?"  Thankfully, the article mentioned that very idea at the bottom.

I wonder what the cost difference would be for stainless vs current grades?  I'm also curious to see what percentage of the total cost of a new bridge project is in terms of just the raw steel used in the project.  I'd assume that raw materials are probably less than 50% of the cost of a typical major infrastructure project such as a bridge?  Purely a guess.  What if the cost of stainless steel is double that of existing grades?  Under that scenario (which again, is probably incorrect numbers), an increase of 100% for the cost of materials which are 50% of a project means that the project as a whole goes up to 150% of the previous cost.  However, if you can double the life expectancy by 2x simply by increasing base cost by 1.5x, you're coming out ahead.  Of course, you also have to calculate NPV, to consider the time value of money invested.

If spending more up front is desirable in terms of reducing overall construction costs, all of our roads would be made of concrete rather than ashphalt.  Concrete is more expensive for road-building, but lasts much longer.  However, I feel that most governments are loath to do what makes sense in the long term if it means a higher budget cost while they're in power.