You only need a smartthings hub if you are going to use zigbee or Zwave devices. If you’re only going to have Wi-Fi, you can operate in a “hub optional“ configuration. That’s because even though the hub does have a Wi-Fi radio, it doesn’t use it to communicate directly to the devices, almost all will all go through the smartthings cloud first. (There are one or two exceptions, but we can discuss that later.) So you don’t add Wi-Fi devices to the hub: you add the integration to your smartthings account through the smartthings app on your phone. 
These days there are many individual brands that have at least some models with official cloud to cloud Integration. For example, TP Link Kasa, Meross, LIFX, Arlo, Leviton’s WiFi devices, August door lock, many more. They don’t necessarily expose all features of a device, but you can build a good system that way. The main thing that is missing is battery operated sensors, but that’s a whole separate discussion.
If the manufacturer doesn’t offer an official integration, there are also some other integration options that may work. See the community FAQ (The topic title is a clickable link):
So, as @nathancu mentioned, it all comes back to the first rule of home automation: “the model number matters.”