He's not a shitty old man. He is going to the extreme (and 10,000 yen obviously doesn't cut it in the real world), but he is teaching his son a lesson, and the lesson is to stop being a spoiled brat.
Takane has a super inflated ego and always buys luxurious things with his money. And yeah, he's rich, but they've talked about how much he's bought since the first chapter. It's money that Takane, if wiser, could store away for later use.
Now if the grandpa wasn't foolish by constantly spoiling his son since a young age, Takane wouldn't be in the mess. But in a way they are both learning a lesson on that and instantly regretting their decisions (old man wishes he didn't spoil his son; son wishes he didn't buy luxurious things almost immediately).
Yep same I'm super angry rn ... like that's way too harsh....
what a piece of shit grandpa. If i was in Takane's place I would throw that table in his house at the grandpa. Shitty old man