I think you hit on the key point. Like you mentioned, you're expecting 3 times as much as you were initially. On top of a better understanding of your own value, you're also going to have a better understanding of the employer:employee relationship. In other words you're not going to bend over backwards solely in in hopes of proving yourself, achieving some sort of recognition, or other such things. I don't think companies are choosing not to hire older developers because of any form of discrimination or even a belief that older developers have less skills. Instead, it's just a value measurement no different than e.g. outsourcing. It's the same reason that I think most large tech companies are pushing hard for 'get [anybody with a heartbeat] into computer science'. Increase the labor supply, lower labor costs, increase profit margins.

This would also explain why other industries don't suffer from a similar problem. In software development companies collect software developers by the tens of thousands. This results in extremely high labor costs and so reductions on this front can see tremendous savings. As an example Google has about 100,000 employees. That means reducing their average salary by just $10 per year is worth a million dollars of 'free' revenue (yeah, ignoring taxes etc). Really amazing to think about those numbers! By contrast even very large companies in other fields will have relatively small numbers of engineers. For instance petroleum engineering is critical in the fossil fuel industry which is an extremely large industry, yet there are only a total of 33,700 petroleum engineers in the entire country.

[1] - https://www.bls.gov/ooh/architecture-and-engineering/petrole...