Yeah, i admit i chose a vid that showcases the ultimate late-game potential of sorceries, but still, you don't need anything other than your basic upgraded Sorcerer's Catalyst to get very decent damage. It's more than enough to carry you all the way to Duke's Archive where you get Big Hat Logan's Catalyst, and finally upgrade. Rickert of Vinheim is accessible from the get-go for more casts of Soul Arrow and the much needed Heavy Soul Arrow, and Griggs of Vinheim is also accessible fairly early into the game. There's nothing really especially hard for a Sorcery build between the start of the game and Lower Undead Burg, and you'll be one-shotting most of the hollows with your casts if you invest into intelligence at all anyway.
When you get down to the Lower Undead Burg, you get Greater Soul Arrow, which will one-shot the things you couldn't one shot before, and Great Heavy Soul Arrow, amazing for bosses. Plus, Bellowing Dragoncrest Ring, for increased sorcery power. After that, if you have the Master Key, you could go to Darkroot Basin, kill the Hydra, save Dusk of Oolacile and get the Crown of Dusk, for another useful boost to your Sorceries power. She also sells the useful Hidden Body and Hush spells, which can be useful when fighting Capra Demon, by making it harder for the dogs to aggro on you.
After Capra Demon, it's smooth sailing right through the end of the game. About the only fight that can give you some trouble is O&S, and maybe Seath because of his annoyingly high magic resistance, but if you have Artorias of the Abyss DLC, Dark Magic will make short work of Seath.
One of the main reasons Magic is so good for PvE is basically because it's ranged. Most of the enemies in DkS will need to get to melee to kill you, and if you kill them before they can, it's GG for them.
In any case, after Duke's Archive, you're incredibly overpowered, especially if you take the time to do the DLC, since you'll have all of the sorceries you could ever want. Even without Power Within and Red Tearstone Ring, you'll find that you don't need all of your castings, or even many castings at all, to make short work of bosses. You can easily maintain your distance, cast when safe, dodge & run when not. And if you want to Melee... Well, Moonlight Greatsword is your friend, you have very high INT already, and it will deal decent damage. Only 11 STR needed to 2-hand it.
Sorcery & Mageblade build has always been my favorite both on DkS and DkS2. I don't think they're any weaker than any other build, but i do admit, it's a build that hinges heavily on having played the game before and knowing where everything is, as well as planning everything out in advance. Not like Havel's Ring + Heavy Armor + Zweihander, where you can get everything in less than 20 minutes into the game, and ROFLSTOMP the content by pumping 3 stats in basically any order you want, no fucks given.