I've had the odd problem with some of the things you're written - but that one takes the biscuit.
Show me some hard,
primary, evidence that Roman Cavalry wasn't just as good as any other cavalry of the 300BC to 300AD period and I'll be happy to debate with you.
Study Xenophon and all the detail of Carrhae (sic) and detail the reasons the Parthians won.
But please don't peddle myths - when there a lot more reasons that even just differences in cavalry quantity, which can often be a factor in military defeats. Please also don't forget that Roman Cavalry helped to re-conquer the West in the 5th century.
Roman Auxiliaries, btw, were fairly responsible for holding the 2nd largest empire the world has ever seen under control for 100's of years. There is precious little evidence that the Roman soldier wasn't damn good throughout the period - but continually let down my amateur leadership and the squabbling of monied-tyrants.
....not much change there then.
In short - for all our period the Romans beat, just about, everybody - and the cavalry were there as part of it.