What are the most basic parts of a bicycle which cannot be upgraded over a period of time?
Certain parts of a bicycle are more easily upgradeable than others, while other types of upgrades require special tools, much more money investments or are limited to whatever standards are used in its design. For some parts, the opportunity of an upgrade coincides with the older part being worn out; in other cases, the replacement is not warranted by this fact.
Let's go from the easiest replacement to the hardest ones. I'll organize them in tiers.

Non-essential stuff that does not affect key function of a bicycle: lights, bells, fenders, phone holders, cycling computers etc. These can be replaced very cheaply and easily.

Contact points: pedals, handlebar grips/tape, saddle. These parts are often a part of one's personal preferences and get swapped immediately after a purchase of a new bike. Many bikes are sold without pedals for this reason, and some people argue the same should apply to saddles and grips.

Regular wear and tear items: chain, tires, tubes, brake pads, cables, bottom brackets. One can upgrade to a lighter chain when replacing a worn one. Tires are often subject to personal preferences the same way as pedals are. One can prefer lighter or sturdier, narrower or wider, smoother or knobbier, all black or tan wall etc. tires depending on what and how they ride. On tubes, one can buy lighter or sturdier ones, or even go tubeless as an upgrade. The same applies to pads and cables: there are aftermarket products which are better in performance/longevity.

Hard wear and tear items: cassettes and chainrings, braking discs. It takes more time, but ultimately, these parts wear out and are bound to be replaced/upgraded. Here compatibility issues are becoming more prominent (even though they already play role in the earlier tiers), and not everything is clearly compatible with everything else. Some parts are likely to be of limited availability. As an example, replacing an 8-speed cassette with 11-speed might bring you to the next tier of costs/troubles, as it will drag everything drivetrain-related with it.

Non-wear components that are at higher risk of breaking: derailleurs, brake calipers and levers, shifters, wheels. Normally these parts do not get worn per se, but they run higher risks of being damaged in minor crashes. Replacement options are often limited by the bicycle design. E.g.: there is no easy way to switch from QR-skewers to through axle hubs unless the frame supports it. Similarly, no safe way to put disc brakes on rim-braked bike.

Non-wear components that are at low risk of breaking: handlebars, cranks, forks, frames, headsets, seatposts. Unless you have a specially nasty hard crash, these parts do not wear out. Replacing them is always justified by the owner's desire to &quot;upgrade&quot; something in any meaning of the word (even if that means changing the color). At this point, the compatibility is the main issue: there are dozens of seatpost diameters and lengths, a hundred of headset dimension combinations, many frame sizes and innumerable variations in geometry, several handlebar diameters and lengths etc. etc. Unless something is explicitly compatible in every aspect of its parameters, it is not.


The presented classification is not universal of course. Certain items can be moved up or down a bit. For example, wheels with rim brake surfaces are rather at level 3 (i.e., eventually worn out). For some people, bottom brackets live for years (thus being on tier 3 or 4), while others replace them every season (tier 2), and so on. I am likely to have forgotten a lot of essential parts as well.