"Why should I buy a computer today, when a computer in 10 years can do a ton more for less money?"
That computer would still feel pretty snappy today for desktop tasks, running dual 2560x1440 screens. Although new AAA games would probably have issues, either low frame rates or not running at all. Most indie games would work just fine.
I think the change will be even less in next 10 years. 2019 hardware will perform just fine in 2029.
Unless something dramatically different pops up, we're in the era of diminishing returns.
For a fixed budget, assuming that Moore's Law still holds (dubious, these days), you should wait until the computation will take 26 months on hardware purchased within that budget [0].