Sorcerers do have the advantages you already noted, spells per day and full spells known always available, as well as access to sorcerer archetypes, arcana, and wild blooded bloodlines. Arcanists can steal some of these by multiclassing and the bloodline development exploit, but that is at the cost of delayed spell casting advancement.
Sorcerers are awesome for their relative simplicity in playing, so it is a great class for first-time full caster players. Don't have to worry about preparing spells (decision paralysis or "wait everyone, I need time to prepare that"), no extra points resource to keep track of (except per day uses of powers), and only have a single key ability for spellcasting and powers.
Arcanists have limited preparation without Quick Study, have to keep track of arcane reservoir points (along with the consume spells/magic items changes), and use INT for spellcasting and CHA for many exploits. Arcanists can also be more limited by campaign design. Like wizards, they require access to spells to make best use of their spellbooks. Consume magic items exploit requires the items to consume (wands, scrolls, potions, or staves). If these are of limited availability during the campaign, the class is weakened relative to a sorcerer.
Arcanists can definitely step on the toes of sorcerers, especially once they clarified runestones to be usable by them to get more spell slots. Sorcerers can make good use of pages of spell knowledge, and can UMD very effectively (SAD focus on CHA), easily using scrolls and wands to expand their versatility. I wouldn't be surprised if someone has done a detailed cost analysis of sorcerers, wizards, and arcanists based on expected item usage over the course of a campaign. Regardless, it often comes down to preferred play style or flavor.
