I think this is an overoptimistic view of the size of the market. The field is highly fragmented. A large fraction of structural engineers are contractors or work for smaller firms (think wood design, rather than concrete/steel) and wouldn't be target customers for section analysis and concrete detailing software. An absolute majority of those that work for the larger firms don't require anything more than Excel spreadsheets. From my experience, a typical structural engineer is a rather savvy computer user, often times writing Excel macros or AutoLISP scripts to automate their tasks.