I'm planning my rig now and am contemplating on the viability of having a long-term PSU solution, together with the casing.

Suppose that I go straight to reputable models like the Seasonic S12G Gold, is it realistic me to foresee using it for the next 5 years and counting? That would mean just like Casing, re-using them even I upgrade my specs 2-3 times in the future?

If a PSU can be something like that, I would consider significantly increasing my budget. If it's something that is 'buy and throw', might as well get Hexa+.

Just to add:
(i) there won't be any OC-ing for this rig. I plan to run stock throughout the years.
(ii) I'll leave at least 20% allowance to total capacity.

Thanks in advance!