I don't think you should budget that way. 8 years is completely unrealistic due to the potential for component failure and Apple's
vintage policy regarding hardware service. I would suggest budgeting based on 3 years of service and get as much additional time out of it as you can.
I just assumed 5-8 years because of my 2006 MBP that I'm still running. Never had any issues other than a couple batteries expanding but it's an easy fix since it's not on the inside of the computer. Just looking for a new one just in case this one dies.
You never know how long a battery will actually last. The one on my 2011 (which is now dying of gpu failure) lasted roughly three and a half years. It had a bit over 500 cycles, and it was dipping below 80% health. It was also expanding. Regardless of cycle count, if the battery is more than a couple years old, I would keep in mind that you're likely to replace it during the time you own that machine. Battery service for the rmbp is $200 in the US if serviced by Apple. It's also a good idea to check the charger for kinks. I've had frayed chargers before. Results vary, but I take my notebook with me quite often. I'm careful with the cord, but their chargers are cheaply manufactured. Also for whatever reason they don't use cord reliefs, which might cut down on fraying.