I have come to the conclusion that a laptop is an inherently unsustainable option: the failure of one part often means the failure of the whole, and they need replacing frequently - a new laptop every few years is one of my bigger impacts on the planet that I can do something about.
When my current laptop fails I am going to transition to a desktop which has a better life expectancy and replaceable parts for the serious stuff and probably look for a reliable tablet for recreational browsing and reading.
Eh, it depends. Better quality laptops will let you replace the most common upgrade components (RAM & storage), and unless you're a serious gamer, the graphics don't need upgrading nearly as often.
Everything else (CPU etc.) tends to be bound by the limitations of the motherboard, regardless of whether it's a desktop or laptop. This is likely to be especially true for Mustachians, who (as with the OP) presumably have longer upgrade cycles/device lifetimes in mind; no six year old desktop mobo would be compatible with current processors, etc., so you'd be replacing mobo, probably RAM, etc. in any case (no pun).
As for peripherals, these days external options offer adequate performance and plenty of selection compared to internal expansion cards. (This is a big change from several years ago, when external busses were often quite limited.)
Pretty much the only thing that's more future-proof about a desktop these days is the case & monitor. Even the power-supply might need updating to match a new mobo's requirements.
...All of that said, if OP's laptop is still working, albeit slow, perhaps they could just hold onto it for travel purposes, while saving money on a new home PC by going the desktop route. It's worth looking into, at least; just don't forget to include the cost of separate keyboard/mouse/monitor if you don't already have those (IMHO worth having even for a home laptop station, just for the ergonomics).