Why is it that expanding space stretches the wavelength of intergalactic light, but doesn't increase its speed?

Isn't the central premise of spacetime that the speed of light is Constant. Such that distance and duration dilate equally?

Instead this theory proposes the wave length is stretched, but the speed is not affected.

What metric is the speed measuring, if not space?

As Einstein said, space is what you measure with a ruler. Given light speed is the most accepted measure of interstellar and intergalactic space, what is it measuring, if not space?

So two metrics are being derived from the same light. One based on the spectrum and one based on the speed. If the speed were the numerator, it would be a tired light theory, but as an expanding space theory, the speed is still being treated as the denominator. The metric by which this expansion is being measured.

Think about all the descriptions of this expanding universe and it is all in terms of how much it grows, relative to the speed, aka, lightyears.

If someone proposed this in 6th grade math, they would get an F.