5-8 years is unbelievably realistic, also illogical, instead of investing for a long term, makes more sense to pay half, and use the other half to buy a better device in 2-3 years (optimistic)
My last MBP was a mid-2009, so that lasted 11 years. I upgraded to 1Tb SSD and maxed the ram to 8Gb and it was still a damn good machine. So good that my neighbor asked to use it during lockdown. If this machine can't last 5 years - half the time - it will be my last Apple laptop.