The only reason I can imagine your cousin is saying that is because you have copper going to the house. Therefore any improvement paying for fiber gets immediately bottlenecked by that, so you are just throwing away money. If however you get fiber to the house, then there is a significant improvement, however this is not always possible.
YouTube and movies are usually always 16:9 or lower vertical resolution, so you will always have horizontal black bars. The only way you can have this without being too annoying is an OLED screen, where the pixels are completely turned off so especially in the dark are not visible. However this comes at an extreme cost