It's not about need. It's about ROI. Does Twitter (or any tech company) _need_ 7500 employees to stay operational? The answer is no.

But let me ask you a different question. Can one engineer increase the companies revenues by 0.1%?

If they can, they just made the company $5m/yr

If the ROI equation of an incremental hire can increase the companies revenues by 0.1% or even 0.01% the right move is to hire as many people as you can while the return on investment holds.