So you're getting 2 for the price of 1.5. Scale that up to 500 devs at a big company and it's a big chunk of change saved on payroll.
Keeping your headcount or hiring humans instead, AI would have to start to cost upwards of $15k/month/developer or more before it costs more than hiring. You're looking at about 4 billion tokens per month before humans start to break even or are cheaper.
But even taking a more realistic 1.25x (20% time savings) gain, lets say you drop from 500 to 400 devs, you'd have to hit around $4,000/dev/month in token spend before hiring humans again would break even.
Payroll is just expensive, in most companies it's by far the biggest expense. AI still has to cost drastically more before investors would call it out as being worse than increasing headcount, from a pure dollars perspective.