Okay, so it would appear that you're the one that doesn't understand networking, because your analogy's terrible.

Here's some issues that you can run into:

If the router's connected with a Cat5 cable, which is very possible given that ISPs give out shitty equipment, it has a max speed of 100Mbps. That speed is shared between all devices connected to the router, so adding more devices means dividing up the speed. Even if it's not a Cat5 cable, your ISP would likely not increase the bandwidth they assign to you for the duration of your hotspot being in use, meaning that the bandwidth is being shared between you and some rando. Furthermore, if you're paying for a really fast connection, you might run into the 1000 Mbps limit of the Cat5e cable, and that would mean that you'd get less than what you're paying for, because, again, you're sharing with anyone connected to the router.

Then there's the issue of wifi interference. Having multiple networks in the same area creates interference, and adding an extra wifi hotspot adds to something that's already a problem in many areas.

Then there's also the issue that yes, it's actually quite easy to hit the capability limits of a cheap consumer router, which is what your ISP's gonna hand out to you. Especially if, as in many homes these days, you have a number of different devices connected up to the router at once, all of them using resources.