No player get's a stuart out to counter a 222. The stuart was and is seen pretty much as a shock unit against infantry in every game. It's indeed hybrid but to say it's role like the AEC, puma or m10 is far fetched, since it has excellent AI abilities unlike those units mentioned.
It still has nowhere near the anti infantry shock value of a T70 or luchs, which can wipe a squad much more quickly and hunt them fown on retreat more effectively.
While people don't buy a stuart to counter a 222, they absolutely do buy it to counter a fht, puma, luchs, ht, or stuka. It is meant to go a lot deeper to fulfill it's roles than a T70 or a luchs, as it's abilities show.
The luchs doesn't need the extra hp to fulfill it's roles, and can make do with micro like the t70 does. It needed it before because of how late it hit, but now it hits with every other light vehicle.
Also, as a side note, the okw has much better supporting at for the luchs than a T70 has, so it's not like the hp is needed to help it survive diving tanks. If the luchs needs 400hp, so does the t70.