The gains aren't even "barely passed on". When new generation chips are released, they'll oftentimes release a new instance type at roughly the same price, then

The gains aren't even "barely passed on". When new generation chips are released, they'll oftentimes release a new instance type at roughly the same price, then re-allocate the older generation chips to power internal services which don't promise a chip family. That internal service might be e.g. S3, so who cares, but it might also be Fargate or Lambda, where you're paying per second of wall time and thus slower chips actively cost you more money.