Alchemy, useless?

At 100 skill and the relevant perks, you can make potions that easily surpass any you'll find or buy, with the possible exception of full heal potions. Add Fortify Alchemy enchantments, and you can easily surpass even those.
Even when you can't surpass their values you can still surpass their performance, because hand-made potions can have multiple effects. For example: Fortify Health + Fortify Two-Handed + Fortify Stamina Regen, which is godly for melee characters. For mages: Fortify Magicka + Fortify Magicka Regen + Fortify Destruction. True, potions that cover one of those effects may have greater values for that effect than the multi-effect ones do, however the fact that they are only covering one of the effects means they're actually markedly inferior in comparison.
You do need a fair number of perks to make good potions, but once you do, the multi-effect ones blow the single-effect ones out of the water, and if you add enchanting on top of that they get insane. Fortify
effects, for example, can, depending on which skill you're boosting, reach +168%, far more than the best found or bought potion.