If you can play both PS2 and PS1 games with the cable and your TV actually displays them, then this drastically reduces your need for things like an OSSC or RetroTINK. You're right about that. Do check with a PS1 game if you haven't already, though.
But even if everything works,you can still get some benefit from one of these devices. It's true that your TV is inevitably going to have to do some upscaling, and TVs are generally not so good at upscaling games (though you should check to see if your TV has a "game" display mode: these modes are far from perfect, but they often help a little). But it is possible, to a certain degree, to get some noticeable benefit out of improving the base image that the TV has to upscale from. In particular, the OSSC and the Tink do a very good job of deinterlacing 480i signals, and deinterlacing is one of the things TVs are especially bad at. So you can get a better quality upscale by offloading the deinterlacing bit to another device.
But component cables do get you most of the way to the best possible image, if they work at all. You're right about that. There are YouTube videos out there that draw comparisons if you'd like to get a more visual representation of what these devices do before making a decision; the "RGB Masterclass" series is especially good.