Do you think that the French were disadvataged in Flanders by not having armoured cavalry (apart from a number of Bavarian squadrons?) in their battles against the Allies?
AFAIK Franch cavalry were armoured - they wore breastplates under the coats. Cuirassiers du Roi were the only regiment to wear breast
and backplates over the coat.
Traditionally, it is assumed that the French cavalry reverted to older tactics between the Nine Years War and the Seven Years War, and that was their downfall.
Not quite, or not by all authors. Closer examination of battles reveal that French horse were at least equal to their Allied counterpart. They won upper hand at Hochstadt, Speyerbach, Almansa, to name only a few engagements they fought.
In others they were beaten by higher tactic - cooperation of Allied infantry and cavalry (Blenheim) or numerical superiority (Ramillies), rather than by force.
At Blenheim, they fought off every effort of (armoured) Imperial cavalry ot the Prince Eugene wing. At Malplaquet they stopped the advance of the whole Allied army, and withdrew only in face of infantry holding breastworks.
The failure of Gendarmerie against English horse is famous, but it's background is unclear - maybe they were desorganized by Hessian fire. We must also remember, that after being "routed" Gendarmerie charged at least six times during the rest of battle, while on the northern wing both sides cavalry gave up after two times.
Generaly, I think that inferiority of French cavalry is a myth, equal only to alleged superiority of "Dutch school" musketry.