You cut off the last part of my statement, which is material to your question.
It seems like your applying broad economic principals to a specific corner of the broader market and, in doing so, you're missing some important nuances. In general I think your ideas make sense, but they're not as easy to apply to structural engineering specifically. If we were workers on an assembly line who can be trained to do a passable job inside of a couple of weeks and probably make a decent wage (for the work), you'd be spot on.
For engineers, the time span to go from "hey, that field is paying well" to "hey, I'm making a decent living at this" is more like 8-10 years. Will there be indirect benefits? Maybe. Some in the heavy civil/transportation sector may get desperate enough to start training building PEs to do the work and pay them enough to convince them to "start over", but that's going to be a very tough sell. We're not all interchangeable cogs in a machine - we have our unique roles that make it difficult to switch back and forth. So a sudden spike in demand for bridge engineers, transportation engineers, or oil and gas engineers would probably take several years to show up in any sort of analytics.
Another important consideration is the unique nature of economies of scale in consulting engineering - and more importantly the diseconomies of scale. There is a sweet spot for every company - dictated by myriad and ever changing variables - between the two. You want to be big enough to handle the right size and quantity of projects to maximize profits, but no bigger as you start incurring more overhead and expenses than you can make up for by increases in revenue. So even though being on the wrong side of that sweet spot can look better because revenues continue to rise, profit margins actually shrink and your overall return on investment falls until the folks at the top are not receiving any additional compensation for growing managerial responsibilities and risk assumption. So smart engineering business leaders in this market shouldn't necessarily be expanding - they should be increasing their fees to increase margins and employee compensation. If they are still below their sweet spot for economies of scale, then expansion and hiring may be a good idea, but it's not always the answer.