Doubtless some of those junior devs will be willing to stick around. In for a penny, in for a pound — very few people want to suddenly take a pay cut; your lifestyle has a way of keeping you hooked on a paycheck of a certain amount, once you get used to it.
And assuming the organization follows a standard "pyramid" model, you don't need that many people to stick around to give a good balance of junior to senior people, and have opportunities for promotion for high-performing/interested junior folks. Lots of companies have to create that attrition artificially; if they get it naturally as a result of people parting ways when their school loans are paid, it's probably solving a problem for them.
Only if everyone that has an opportunity to work there shares your moral compass. This industry is really good at marketing, and not just to customers but employees too.
I mean, look at how popular facebook still is among the public even after all of these scandals. There doesn't appear to be any serious attempt by the shareholders to demand change, and Zuck always seems to escape responsibility. I don't remember the exact number, but someone posted data on HN showing that a high percentage of current employees at FB (far more than 50%) see the company as still providing a net positive impact on the world.
1999,MS Windows was de facto, but everyone hated it. Linux/Unix/MacOS was for the cool kids.
But all business folk loved MS, still do, from shareholders to 401k holders.
FB is no different.
I don't see myself (not debt-laden, have a great job) compromising mine just to work for them.
What's the long term effect of that trend? Will FB be chock full of juniors with an ever depleting number of seniors?