Er, OK, but I haven't worked out what you're disagreeing with.
I should expect Vy to decrease with altitude in terms of IAS. But what should happen to Vy in TAS terms? I see no reason why a plane whose thrust does decrease with speed and altitude, but does so only slowly (low-bypass turbofan, afterburner...) could not have TAS Vy increasing all the way to its ceiling where it is overtaken by Vx?
Sure, but V-speeds are IAS (or CAS) not TAS. The TAS corresponding to Vy may well increase. In the case of a constant thrust jet, both power required and power available have a dependence on the TAS, so Vy may remain constant in EAS, with the TAS corresponding to it increasing with altitude.