Isn’t the PC the cheaper option? You act as if the only benefit you get is the upgradeability, but since a desktop doesn’t have to be as small, the components are much cheaper individually, which makes a PC cheaper than a Steam Deck simply looking at performance.
It’s not a necessity to upgrade the PC, and if you never upgrade it, then it also never costs more than the Steam Deck.
In essence upgradeability should be almost not a decision factor, since you probably can “upgrade” the Steam Deck in the future as well: you buy a new Steam Deck handheld once Steam releases a more powerful version. Yeah, you’re not swapping out components, but there’ll very likely be a way to copy over your setup/data, and then the only difference is that the upgrade is more expensive, same as the initial purchase is right now.