The only people who thought that 4e's profits were "running on fumes" worked for WotC/Hasbro.
Much like 2E, 3 and 3.5, there was a spike in sales when first released that then decreased. The devs for 5E thought they were working on the last set of books that would ever be released.
As far as whether the people that actually saw sales numbers were the "only ones", I simply disagree.