A decently fast laptop can definitely replace an older desktop computer without issue. When my old gaming rig died in 2017, I bought a laptop for work and plugged it in my monitors. Apart from graphics, the laptop was measurably faster on all accounts.
The only machines I've compared were my old gaming rig (with various parts bought or upgraded anywhere between 2009 to 2012-13) and my 2017 laptop. Not sure where that 3 years come from, maybe I'm misunderstanding.
