After reading this question: Cheap MTB with future upgrades v/s costly fully ready MTB, I myself had a question.
What are the most fundamental parts of a bicycle? What are the things which we have to look for in a bicycle prior to buying which needs to/can be changed over a period of time but cannot be upgraded? I know we can change derailers, drivetrains, and stuff but is there a part in a Bicycle that cannot be changed/upgraded over a period of time?
I am not the best to phrase a question so to further explain my question, I'll try to provide few examples.
Example #1: There are two cycles - Cycle A's frame is made of Steel and Cycle B's frame is made of Aluminium. We know that frames made of Aluminum are generally stiffer than steel which saves few fractions of a second count. Let's say I bought a Steel Framed Bicycle and want to make it more track-oriented and save a few microseconds off. I cannot have a stiffer frame because I cannot upgrade the material of the frame. If only someone would have told me that I should have invested more on a bicycle which had an Aluminium frame because I cannot change it.
Example #2: I want to install a larger size of rims from 26" to 27.5" because Larger wheels hold more speed than smaller wheels due to rotational inertia and more grip because the larger wheel has more rubber touching the ground. Let's say my suspension is not allowing me to fit the larger wheels which would not allow me to harness the advantage of more speed and grip. If only someone told me to get a cycle with bigger wheels.
Example #3: I want to upgrade the derailer from 24 to 27. I know it's possible, thanks to Bicycle Stack Exchange, I got good guidance for the upgrade and got lucky with the fact that my cycle allowed me to upgrade the derailer. If my cycle didn't have space for a 27-speed crank then it would not have been possible for me to upgrade. Let's say I bought a 21-speed bicycle because it's cheaper then I mostly wouldn't have had any space for a 27-speed crank or even a 24. I would have been stuck with my 21-speed until I buy a new cycle.
I am so sorry if you find these examples very silly. I made up these examples and personally I don't have both the issues. My bicycle is made of Aluminium, has space for a 27.5" upgrade and has a newly upgraded 27-speed derailer, crank, and shifters but I am a noob when it comes to cycling and who genuinely wants to ask this question. Are there any other factors such as these which I should consider before buying a new cycle so that I won't regret it?
I am trying to curate a list of these factors from your expertise which needs to be considered before buying a bicycle to prevent anyone from making a mistake that cannot be rectified in the future. I know I did a bad job explaining the question so edits are happily welcomed.