Difficulty set by pools Can we argue that, since the difficulty set by pools is lower than the actual one, "old" hardware remain profitable for longer and thus are used for more time. And this, indirectly, increases the actual hash rate of the entire network? (and thus difficulty as well?)
No, you see the difficulty of a pool is lower in order for the pool to be able to verify that the miners are actually working on a solution. The shares a miner obtains are mostly not valid blocks, which is what gets rewarded in Bitcoin, paid out to the pool and then paid to the miners.

The difficulty to create a block is not influenced by a pool's share difficulty, hence the total payouts the pool makes does not change, and old hardware is just as (un-)profitable.