Infantry switch from phalanx to line infantry then to modern troop fighting boots on the ground in small team, still holding the lines... Nobody ask "when infantry disappear"?
Same for cavalry, cavalrymen never disappear, they adapt. Horse transform into trucks, tanks .... But they role stay on: recon, flank coverage and breaking enemy lines by frontal attack.
In several conflict they were less efficient, wwi for example, but they are still here.
In a lot of armies, the armored force became a separate part, but in France, we still have cavalry:
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Cuirassier: heavy armored soldier are now in Leclerc MBT, but same role, heavy hitting, shinny and expensive
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Hussards: light horsemen for recon, are still reconnoitring ennemy lines (see the 2nd and 3rd hussard, on VBL and AMX-10RC), some are even paratroopers, the 1st RHP on ERC-90
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Spahis on AMX-10RC
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Chasseurs and
chasseurs à cheval: AMX-10RC
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Infantrie de marine and
Infanterie Char de marine: maritime infantry cavalry, yes we have this! on AMX-10RC and ERC-90
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Dragons : on regiment for the special force (13th RDP) for very deep recon, one regiment (4th RD) on Leclerc MBT, one ( 2nd RD) specialised in NRBC combat and recon
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501 regiment de char (tank regiment): formed in 1918 as a specialist regiment to use the new tanks, the cavalry keep it. Now on leclerc MBT
As you see, cavalry keep almost the same missions, but evolve in his material and tactics. They obtain some specialised missions (deep recon, NRBC...) cause they are more intelligent than infantrymen
ps: the best cavalrymen evolve in an even superior species: the pioneer and the artillerymen