vx & vy converge at altitude

On Monday I am participating in a density altitude seminar, and I have
been tasked with explaining why vx and vy converge at altitude.
The explanation needs to be simple - i.e. no algebra calculations etc -
just stripped to the bare bones.

Here is what I have come up with.

VX utilises excess thrust.
VY utilises excess thrust hp.

As density altitude increases, VX angle of attack becomes lower,
VY angle of attack increases.
Somewhere between service ceiling and absolute ceiling they converge.

Can anyone explain it simpler (and/or more accurately in as few words)
than that?

Thanks

Tony
--

Tony Roberts
PP-ASEL
VFR OTT
Night
Cessna 172H C-GICE