A couple things make your scenario less likely than you might believe.
1. Salaries rarely go down once you've landed the job. So if he can get an $70k job now and get a $5k raise each year for the next 4 years that puts him in a pretty good spot to switch jobs and jump to the $100k.
2. Depending on his desire he could leverage his programming + design skills in a unique way that allows him to get to $100k from freelancing, building products, etc. Learning the backend is a stepping stone not the destination, but combined with #1 above means earning an extra $20k on the side is very doable.
3. I doubt that the ability to build enterprise applications out of ruby/python/js will ever be seen as bottom tier office work. People can't figure out how to work their own printers at work, I don't think we'll see these people suddenly learning how to build enterprise applications. Again, even if that is the case, sit tight and your salary probably won't decline as long as you don't leave the job.